The Science of Sauna and Cold Plunge: Evidence-Based Health Benefits

Abstract

Background: Sauna bathing and cold-water immersion (cold plunging) are age-old practices that have recently gained popularity for purported health benefits. Both represent thermal stressors that may induce adaptive physiological changes (hormesis) with implications for cardiovascular, metabolic, neurocognitive, immune, and recovery-related health. Objective: To comprehensively review and synthesize evidence on the health effects of sauna use and cold-water immersion, independently and in combination (contrast therapy). We prioritized high-quality studies (systematic reviews, meta-analyses, randomized trials) to evaluate outcomes across cardiovascular, metabolic, neurocognitive, immune, and exercise recovery domains, and to elucidate underlying mechanisms (e.g. endothelial function, brown adipose tissue activation, heat shock protein induction). Methods: A literature search was conducted for peer-reviewed studies on sauna bathing and cold-water immersion (2000–2025). Data from systematic reviews, meta-analyses, longitudinal cohort studies, and randomized controlled trials (RCTs) were extracted. Key findings were tabulated, and effect sizes (hazard ratios, risk reductions, or standardized mean differences) are reported where available. Results: Regular sauna bathing is associated with improved cardiovascular profiles (including lower blood pressure and arterial stiffness) and robust epidemiological links to reduced risks of fatal cardiac events, stroke, dementia, and all-cause mortality . Sauna use also correlates with better mental well-being, improved sleep, and fewer respiratory infections . Cold-water immersion acutely triggers significant physiological responses (e.g. sympathetic activation, inflammation) but may confer longer-term benefits such as reduced stress, enhanced immune resilience (29% fewer sick days with routine cold showers) , improved insulin sensitivity, and alleviation of depressive symptoms . Post-exercise cold-water therapy shows efficacy in reducing muscle soreness and biomarkers of muscle damage, aiding recovery . Mechanistically, heat exposure induces heat-shock proteins, anti-inflammatory pathways, vasodilation, and improved endothelial function, whereas cold triggers catecholamine release, brown adipose tissue activation, and alternate vasoconstriction/vasodilation cycles that may improve vascular tone . Conclusion: Evidence suggests that both sauna bathing and cold-water immersion can impart diverse health benefits – sauna use has strong observational links to reduced cardiovascular and neurodegenerative risk, while cold immersion shows promise for stress reduction, metabolic improvements, and exercise recovery. Combined hot/cold contrast therapy may further vascular conditioning but requires caution. The overall quality of evidence varies, with many observational associations for sauna and fewer RCTs for cold immersion. Future high-quality trials are needed to confirm causal benefits, optimize protocols, and clarify mechanisms. Keywords: Sauna, Cold-Water Immersion, Contrast Therapy, Cardiovascular Health, Metabolic Health, Neurocognitive Function, Immune Function, Exercise Recovery, Heat Shock Proteins, Brown Adipose Tissue.

Introduction

Sauna bathing and cold-water immersion have been embraced in cultures worldwide for centuries – from the Finnish sauna to Nordic ice swimming – as means of promoting health, relaxation, and community bonding . These practices subject the body to thermal extremes: saunas involve exposure to high heat (often 80–100 °C in a dry Finnish sauna) inducing profuse sweating, whereas cold-water immersion entails brief exposure to very cold water (typically ≤15 °C) causing an intense cold shock response . Both are considered forms of hormetic stress – short, controlled stressors that trigger adaptive responses leading to stronger physiological resilience over time . As such, there is growing scientific interest in their potential health benefits and therapeutic applications.

Sauna Bathing: Modern epidemiological research from Finland (where sauna use is routine) has brought sauna bathing to the forefront of preventive health discussion. Frequent sauna use has been correlated with reduced cardiovascular and all-cause mortality, lower risk of neurocognitive diseases, and other positive health outcomes . Mechanistically, heat exposure in a sauna elevates core body temperature and activates thermoregulatory pathways via the hypothalamus and autonomic nervous system . This leads to a cascade of physiological effects: heart rate increases, blood vessels dilate (especially in the skin), blood pressure initially rises then often falls post-sauna, and sweating promotes fluid loss . On a molecular level, heat stress induces heat shock proteins (HSPs) and other cytoprotective proteins, reduces oxidative stress, and improves nitric oxide (NO) availability, enhancing endothelial function . These changes mirror some benefits of moderate exercise, earning sauna the moniker “passive aerobic exercise” in some literature . Sauna bathing is also reported to modulate the autonomic nervous system (shifting toward parasympathetic dominance after use) and stimulate the release of endorphins, contributing to stress relief and relaxation. Culturally, sauna sessions are often followed by cooling-off periods (sometimes including cold plunges) and rehydration, practices thought to further accentuate circulatory benefits .

Cold-Water Immersion: In contrast, cold-water immersion (CWI) or cold plunging exposes the body to abrupt cold stress, provoking a strong sympathetic “fight-or-flight” response. Upon immersion in icy water, skin temperature drops and cold thermoreceptors trigger instantaneous effects: vasoconstriction (narrowing of blood vessels) in the skin and extremities, a surge of catecholamines (norepinephrine and epinephrine), increased heart rate and blood pressure, and a gasp reflex followed by hyperventilation . These acute responses carry inherent risks (e.g. arrhythmias or shock in susceptible individuals) but also underlie many proposed benefits through a hormetic mechanism. Repeated cold exposure is believed to stimulate brown adipose tissue (BAT) activation – brown fat being a thermogenic tissue that burns calories to produce heat – thereby improving metabolic health by increasing energy expenditure and insulin sensitivity . Cold immersion also prompts the release of neurotransmitters and mood-regulating hormones: for example, brief cold exposure increases blood norepinephrine levels by two- to threefold, which can reduce inflammation and act as an analgesic, and β-endorphin and dopamine levels also rise, potentially improving mood and cognitive focus . Anecdotally, many people report feeling invigorated, euphoric, and mentally clear after a cold plunge – effects sometimes described as an endorphin “high” or increased alertness akin to a strong cup of coffee .

Rationale and Scope of Review: Emerging scientific studies have begun to quantify these benefits and explore therapeutic uses of sauna and cold exposure. Notably, frequent sauna bathing has been examined as a lifestyle factor influencing cardiovascular longevity, while cold-water therapy is being investigated for its anti-inflammatory and mental health effects (popularized by practitioners of the “Wim Hof method” of cold exposure and breathing). Additionally, contrast therapy – alternating between hot sauna sessions and cold plunges – has garnered attention in sports and wellness communities for potentially synergizing the benefits of both modalities. However, despite growing public enthusiasm, the medical evidence remains fragmentary: sauna studies span observational cohorts and small trials with heterogeneous endpoints, whereas cold-water immersion research is relatively nascent and often limited to short-term outcomes or specific populations (e.g. athletes or winter swimmers). Safety considerations also merit scrutiny, as improper use of these practices can lead to adverse events (dehydration, hypotension, arrhythmias, or, in the case of cold exposure, hypothermia and drowning risk) .

This review aims to provide a comprehensive meta-analytic overview of the health benefits (and risks) of sauna bathing and cold-water immersion. We synthesize findings across multiple health domains – cardiovascular, metabolic, neurocognitive, immune, and recovery from exercise – and evaluate the strength of evidence supporting each. We also discuss the biological mechanisms identified in experimental studies, such as improvements in vascular endothelial function, induction of heat shock and cold shock proteins, modulation of autonomic tone, brown fat recruitment, and changes in inflammatory pathways. In addition, we consider the combined use of sauna and cold exposure (contrast hydrotherapy), examining whether alternating extreme temperatures confers additive benefits or heightened risks. Ultimately, our goal is to present a balanced, rigorous analysis of how these thermal therapies influence human health, highlight gaps in current knowledge, and propose directions for future research. By targeting a general but analytically minded audience, we explain scientific findings in accessible language while maintaining the nuance and detail needed to appreciate this evolving field.

Methods

Literature Search and Inclusion Criteria

We conducted a systematic search of the biomedical literature (PubMed, Web of Science, Scopus) for studies published from 2000 through 2025 pertaining to health effects of sauna bathing (including traditional dry Finnish saunas and infrared saunas) and cold-water immersion (including cold baths, ice swims, and cold showers). Search terms combined keywords like “sauna”, “thermal therapy”, “passive heat”, “cold water immersion”, “cold exposure”, “ice bath” with outcome-specific terms (e.g. “cardiovascular”, “blood pressure”, “mortality”, “cognitive”, “mood”, “immune”, “inflammation”, “muscle recovery”). We also reviewed reference lists of relevant articles (including prior reviews) for additional sources.

For sauna bathing, we included randomized controlled trials (RCTs), observational cohort studies, and meta-analyses/systematic reviews that assessed health outcomes or physiological effects in humans engaged in regular sauna use. For cold-water immersion, given fewer long-term studies, we included RCTs, controlled experimental studies, and systematic reviews examining acute and chronic effects of cold plunges or winter swimming on health-related outcomes. We prioritized higher-level evidence: systematic reviews and meta-analyses were given greatest weight, followed by large prospective cohort studies and RCTs. Case reports and anecdotal evidence were generally excluded unless particularly illustrative of a mechanism or under-researched area. Non-English language studies were excluded unless an authoritative English summary was available.

Data Extraction and Synthesis

For each included source, data were extracted on study design and population (sample size, demographics), intervention or exposure details (sauna temperature and session duration/frequency; water temperature and immersion duration/frequency for cold exposure), comparison condition (e.g. control group or lower exposure category), and key outcomes measured. Outcomes of interest were grouped into five broad domains, reflecting the areas specified in our review objectives:

• Cardiovascular outcomes: e.g. blood pressure, arterial stiffness, endothelial function, incidence of hypertension, cardiovascular disease (CVD) events, and cardiovascular mortality.

• Metabolic outcomes: e.g. insulin sensitivity, blood glucose control, lipid profiles, body composition (fat mass, particularly brown vs. white adipose tissue), and incidence of metabolic diseases (type 2 diabetes, obesity).

• Neurocognitive outcomes: e.g. cognitive function, risk of dementia/Alzheimer’s, mood and mental health metrics (symptoms of depression or anxiety), and cognitive performance or brain imaging changes.

• Immune and inflammatory outcomes: e.g. frequency of infections (common colds, pneumonia), markers of immune function (white blood cell counts, immunoglobulin levels), systemic inflammation markers (C-reactive protein, cytokines), and reports of autoimmune or anti-inflammatory effects.

• Exercise recovery and physical performance: e.g. post-exercise muscle soreness (DOMS), recovery of strength/power, markers of muscle damage (creatine kinase, lactate dehydrogenase), and athletic performance outcomes after sauna or cold interventions; also perceived recovery and fatigue.

We tabulated key findings from representative high-quality studies for sauna and for cold exposure, including effect sizes such as hazard ratios (HR) for disease outcomes or standardized mean differences (SMD) for pre-post changes, where reported. Meta-analytic summary estimates were recorded when available. No formal meta-analysis was performed by us on primary data; rather, we qualitatively synthesized results across studies, noting consistencies, discrepancies, and the degree of confidence supported by the evidence (taking into account sample sizes and risk of bias as reported in original studies or systematic reviews).

Quality and Bias Assessment

We relied on published assessments of study quality when available (e.g. Cochrane risk-of-bias for RCTs or AMSTAR ratings for reviews). For cohort studies, we considered factors such as adjustment for confounders and follow-up duration. Given that many sauna studies are observational, we paid attention to potential confounding (for instance, sauna users might differ in lifestyle from non-users) and reverse causation (e.g. healthier individuals using saunas more). In cold exposure studies, randomization and blinding are challenging (participants obviously know they are in cold water), so we noted when outcomes could be subjective (e.g. mood ratings) and susceptible to placebo effects. We also highlight gaps where evidence is thin or findings are preliminary. This narrative approach, grounded in systematically identified evidence, provides a meta-analytic style integration of current knowledge rather than a new quantitative meta-analysis.

Results

Overview of Included Studies

Our search yielded a diverse body of literature. For sauna bathing, evidence is bolstered by several large Finnish prospective cohort studies (with thousands of participants followed for 10–20 years) and a number of small-to-moderate RCTs (typically under 100 participants) examining clinical endpoints like blood pressure, heart failure symptoms, or exercise tolerance. We identified at least two major systematic reviews summarizing sauna’s clinical effects . For cold-water immersion, research included recent systematic reviews and meta-analyses focusing on outcomes like inflammation, stress, and post-exercise recovery , as well as numerous small experimental studies on immune changes, mood, and metabolic responses, often involving experienced winter swimmers or athletes. Table 1 and Table 2 summarize key findings from representative high-quality studies on sauna and cold-water immersion, respectively.

Table 1. Key Studies on Sauna Bathing and Health Outcomes

Study (Year)	Design & Population	Outcomes Measured	Key Findings
Laukkanen et al., 2015 (Finland)	Prospective cohort (20.7-year follow-up) of 2,315 middle-aged men	CVD mortality, sudden cardiac death (SCD), all-cause mortality	Dose-dependent risk reductions with higher sauna frequency. Men taking 4–7 saunas/week had 63% lower risk of SCD (HR 0.37, 95% CI 0.18–0.75) and ~50% lower fatal CVD/CHD events versus one sauna/week. All-cause mortality ~40% lower. Sessions >19 min linked to ~52% lower SCD risk than <11 min.
Laukkanen et al., 2017 (Finland)	Prospective cohort (20-year) of 2,315 men (same cohort as above)	Incidence of dementia and Alzheimer’s disease	Neurocognitive benefit: 4–7 saunas/week → ~66% lower dementia risk (HR 0.34, 95% CI 0.16–0.71) and ~65% lower Alzheimer’s risk (HR 0.35) vs one sauna/week.
Kunutsor et al., 2017 (Finland)	Prospective cohort (median 25 years) of ~2,200 middle-aged men	Incidence of respiratory diseases (pneumonia)	Immune/respiratory: 4–7 saunas/week → 41% lower pneumonia risk (HR ~0.59) vs ≤1/week. 2–3 saunas/week → 27% reduced pneumonia risk. Suggests dose-response via immune surveillance + reduced oxidative stress in lungs.
Hussain & Cohen, 2018 (Systematic Review)	Systematic review of 40 sauna studies (13 RCTs; total ~3,855 participants)	Blood pressure, heart function, pain, etc.	Regular dry sauna bathing has broad health benefits, minimal adverse events. Improved BP and arterial compliance, enhanced cardiac function in CHF, reduced chronic pain, improved exercise tolerance. One small RCT: reversible sperm count reduction in men.
Laukkanen & Laukkanen, 2024 (Comprehensive Review)	Narrative review of passive heat therapies (esp. Finnish sauna)	Epidemiological + mechanistic findings on healthspan	Sauna use consistently correlates with reduced risk of hypertension, CVD, dementia, respiratory illness. Also improved musculoskeletal symptoms, fewer colds/flu, better mental wellbeing and sleep. Mechanisms: anti-inflammatory, antioxidant, circulatory, immune modulation.

Key: HR = hazard ratio; CI = confidence interval; CVD = cardiovascular disease; CHD = coronary heart disease; RCT = randomized controlled trial.

Study (Year)	Design & Population	Outcomes Measured	Key Findings
Cain et al., 2025 (Systematic Review & Meta-Analysis)	11 studies (7 moderate-, 4 high-quality RCTs; N = 3,177 healthy adults)	Acute and short-term effects of CWI on stress, mood, inflammation, immunity, sleep	CWI (≤15 °C, 30 s–20 min) acutely ↑ inflammation markers but ↓ stress by 12h (SMD –1.0). Daily cold showers led to ~29% fewer sick-leave days. Some studies show better sleep and QoL, though protocols varied.
Buijze et al., 2016 (Netherlands RCT)	~3,000 working-age adults, randomized to daily hot-to-cold showers vs. control	Sickness absence, energy, quality-of-life	Cold shower groups had ~29% reduction in sick leave vs. control. No reduction in illness incidence, but milder symptoms. 91% of participants kept up cold showers voluntarily after study.
Janský et al., 1996 (Czechia)	Controlled cold adaptation trial in healthy adults	Immune markers, stress hormones	Regular cold exposure ↑ WBC counts, improved tolerance to cold, attenuated cortisol spikes. Some evidence of ↓ baseline TNF-α. Fewer respiratory infections observed in adapted group.
Yankouskaya et al., 2023 (UK)	Pre/post experimental study (39 adults, 5 min immersion at 20 °C)	Mood (PANAS) and fMRI brain connectivity	Significant ↑ in positive affect and ↓ in negative affect post-CWI. fMRI showed increased connectivity in salience/attention networks. Authors attribute benefits to ↑ norepinephrine, dopamine, serotonin.
Xiao et al., 2023 (Meta-Analysis)	20 RCTs/crossover trials of post-exercise CWI	Muscle soreness, CK, lactate, inflammation, subsequent performance	CWI (10–15 °C, ~10 min) ↓ DOMS and fatigue, ↓ CK and lactate 24–48h post-exercise. No consistent change in CRP or IL-6. Helped preserve power in some studies, but frequent CWI may blunt long-term hypertrophy.

Key: CWI = cold-water immersion; RCT = randomized controlled trial; DOMS = delayed onset muscle soreness; CK = creatine kinase.

Cardiovascular and Circulatory Effects

Sauna Bathing – Cardiovascular Health: The cardiovascular system is profoundly affected by sauna’s heat stress. During a sauna session, heart rate often rises to 120–150 beats per minute (similar to moderate exercise) and cardiac output increases by 60–70% as blood flow redistributes toward the skin for cooling . Repeated sauna use appears to condition the cardiovascular system. In hypertensive individuals, sauna sessions cause an initial blood pressure elevation followed by a longer post-sauna reduction due to vasodilation and improved arterial compliance. Over time, regular sauna bathing has been associated with lower blood pressure and reduced risk of developing hypertension. For example, a Finnish cohort study reported that men taking saunas 4–7 times weekly had significantly lower risk of incident hypertension over 10 years compared to those with one sauna or none . Mechanistic studies suggest sauna improves endothelial function – heat acutely raises nitric oxide (NO) levels, which dilate blood vessels, and repeated exposure may enhance endothelial responsiveness. In one trial with patients with at least one cardiovascular risk factor, two weeks of daily 30-min sauna sessions improved flow-mediated dilation (a measure of endothelial health) and reduced arterial stiffness indices . Heat conditioning also stimulates angiogenesis (formation of new blood vessels) and increases circulating endothelial progenitor cells, potentially aiding vascular repair . Additionally, sauna-induced sweating leads to a fluid shift that can resemble the effects of a mild diuretic, which may help reduce blood pressure and cardiac workload.

Perhaps the most striking evidence comes from long-term observational studies. As shown in Table 1, the frequency of sauna use is inversely associated with fatal cardiovascular outcomes. Men who used a sauna nearly every day (4–7 times/week) had ~50%–60% lower rates of fatal heart disease or sudden cardiac death than once-weekly users in the Kuopio Ischemic Heart Disease Study . Notably, this relationship held even after adjusting for numerous confounders (age, smoking, blood pressure, cholesterol, exercise habits, etc.), suggesting an independent benefit of sauna bathing . The duration of each sauna session also mattered: sessions longer than 19 minutes were linked to significantly fewer cardiac events than shorter sessions . These findings point to a dose-response protective effect. It is hypothesized that regular exposure to heat stress acts like a passive cardiovascular workout – improving hemodynamics, promoting favorable blood lipid profiles, and reducing chronic inflammation (systemic inflammation markers like C-reactive protein tend to be lower in frequent sauna users) . Furthermore, sauna use is often part of a relaxing routine that might reduce stress hormone levels; chronic stress is a known cardiovascular risk factor, so this relaxation component could also contribute indirectly.

Sauna therapy has even been tested in clinical populations: patients with chronic heart failure (CHF) can benefit from carefully monitored sauna sessions (often called “Waon therapy” in Japan). In one study, CHF patients who underwent 4 weeks of daily 15-minute, 60°C sauna sessions showed significant improvements in heart function (ejection fraction increased from 24% to 31%), reduced pulmonary artery pressure, and improved exercise tolerance without adverse events . Improved endothelial function (via increased NO and decreased oxidative stress) is posited as a mechanism for these benefits in heart failure . Professional guidelines now cautiously endorse sauna bathing as a complementary therapy for stable CHF and peripheral artery disease, provided patients are well-supervised . However, sauna is contraindicated in severe aortic stenosis, unstable angina, or immediately after acute myocardial infarction . For most individuals with stable cardiac conditions, sauna appears safe (the incidence of acute cardiac events in the sauna is very low – Finnish data showed only ~1–2% of heart attacks occurred during or soon after sauna use) . Proper hydration and avoiding alcohol (which can exacerbate hypotension and arrhythmias in the heat) are important safety measures .

Cold-Water Immersion – Cardiovascular Effects: Acute cold exposure has essentially opposite immediate effects to heat: it causes peripheral vasoconstriction and a spike in blood pressure. Upon immersion in cold water, systolic blood pressure can rise by 20 mmHg or more and the heart rate initially jumps due to sympathetic discharge (the “cold shock” response). In people with normal cardiovascular function, these responses are generally well tolerated and transient; in fact, repeated cold exposure can lead to beneficial adaptations. For instance, studies of winter swimmers have found that while novice individuals show large blood pressure and heart rate increases when first plunging into near-freezing water, experienced cold swimmers exhibit blunted cardiovascular responses – an indication of improved autonomic control and tolerance . Regular cold immersions tend to raise basal vagal tone (parasympathetic activity) at rest and may improve heart rate variability, a marker of cardiovascular health . One meta-analysis reported that cold water or whole-body cryotherapy can enhance parasympathetic activity during recovery from exercise, which could benefit the heart by reducing strain .

Chronic cold exposure might also improve circulation by fostering a more flexible vascular system. Alternating vasoconstriction (in cold) and vasodilation (after rewarming) effectively gives blood vessels a “workout,” potentially improving their elasticity. Contrast hydrotherapy practices (hot-cold alternation) are based on this principle. A small crossover study found that a series of alternating infrared sauna and cryotherapy sessions improved forearm blood flow more than traditional warm-water immersion therapy . It appears that oscillating between high and low temperatures trains the endothelium to respond efficiently to changing demands, thereby enhancing overall vascular reactivity. Some researchers even describe an “endothelial pump” effect: during heat, vessels expand and fill with blood; during cold, they constrict and force blood back to the core – this pumping action may help flush out metabolic byproducts and improve capillary function .

However, unlike sauna, there is limited long-term epidemiological data on cold exposure and hard cardiovascular outcomes. One notable theme is metabolic-cardiovascular crossover: because cold immersion can improve insulin sensitivity and lipid profiles (see next section), it could indirectly benefit cardiovascular health by reducing atherosclerotic risk factors . The 2022 systematic review by Espeland et al. concluded that CWI “may have a protective effect against cardiovascular and metabolic diseases” but also emphasized that many studies were small and not focused on clinical endpoints . There are intriguing case studies of cold-adapted individuals with exceptionally high blood adiponectin (a cardioprotective hormone from adipose tissue) or low C-reactive protein, hinting at anti-atherogenic effects. Additionally, cold exposure has been shown to increase blood HDL cholesterol in some experiments (possibly via BAT activity burning triglycerides) , and to acutely raise blood pressure in a way that might paradoxically train the baroreceptors to better stabilize blood pressure over time.

Safety considerations: For individuals with known heart disease or uncontrolled hypertension, the rapid blood pressure elevation from cold immersion can be dangerous – there are case reports of arrhythmias or even cardiac arrest triggered by cold shock in people with underlying coronary disease. Thus, medical guidance is advised for such individuals before engaging in cold plunges . For the general healthy population, brief cold exposures are usually safe, but acclimatization is important: starting with milder cold or shorter duration to allow the body to adapt. Similarly, dehydration and alcohol should be avoided prior to cold exposure, as they can impair the body’s response (alcohol, for example, paradoxically causes surface vasodilation, increasing heat loss and risk of hypothermia).

In summary, sauna bathing shows strong evidence of cardiovascular benefits, from improved blood pressure and vascular function in clinical studies to dramatically lower cardiovascular mortality in long-term cohort data . Cold-water immersion exerts acute cardiovascular stress but may lead to adaptations that improve circulatory efficiency and autonomic balance. Both practices, when used appropriately, can complement heart-healthy lifestyles – sauna by providing passive cardio conditioning and stress reduction, and cold immersion by metabolic activation and possibly strengthening vascular reflexes. Nevertheless, individuals with cardiovascular conditions should approach these therapies with caution and ideally under supervision.

Metabolic and Endocrine Effects

Heat and Metabolic Function (Sauna): Sauna use can influence metabolism in several ways. The immediate effect of sitting in a hot sauna is an increase in energy expenditure – one sessions can burn roughly 150–300 calories in 30 minutes for an average adult, due to the energy cost of sweating and maintaining core temperature. This is modest (akin to brisk walking) but over time regular sauna baths might aid weight management slightly. More significantly, sauna’s heat stress has been linked to improved insulin sensitivity. Heat exposure triggers the release of heat shock proteins (HSPs) which have been shown to enhance insulin signaling pathways and glucose transport in muscles . In one study on obese adults, repeated heat therapy (hot tub bathing) over 3 weeks lowered fasting blood glucose and insulin levels, suggesting better glycemic control . Sauna bathing, similarly, has been associated with lower risk of developing type 2 diabetes in observational research: in a Finnish analysis, high-frequency sauna users had a significantly lower incidence of diabetes over the long term compared to infrequent users, even after accounting for physical activity and BMI (though confounding cannot be entirely ruled out).

Furthermore, sauna sessions transiently raise growth hormone (GH) levels – in some studies, GH spiked by 200–300% after a 20-minute 80°C sauna, which could have metabolic implications for muscle maintenance and fat oxidation. They also elevate circulating adiponectin, a hormone that increases fatty acid oxidation and is linked to insulin sensitivity. Combined with the slight cardiovascular workout effect, these hormonal changes form a picture in which sauna use mimics some aspects of exercise on metabolism. For individuals who cannot engage in intense physical activity (due to joint pain or other issues), sauna might serve as a supportive therapy to improve metabolic health .

Regular sauna bathing has been noted to reduce systemic inflammation – chronic low-grade inflammation is a driver of insulin resistance and metabolic syndrome. In a cross-sectional study, those using saunas most frequently had the lowest levels of C-reactive protein and IL-6 . Heat shock proteins induced by sauna can directly interfere with pro-inflammatory signaling. By lowering inflammation, sauna could alleviate one root cause of insulin resistance. Additionally, improved blood flow from heat means better nutrient delivery and waste removal in tissues, possibly benefiting metabolic function at the cellular level.

From a clinical standpoint, small trials have experimented with sauna or Waon therapy for patients with metabolic syndrome or obesity. Some report mild weight loss (likely mostly water loss acutely, but possibly a small reduction in fat mass over time) and improved cholesterol profiles. For example, in one trial of twice-weekly sauna for 3 months, participants saw a decrease in total and LDL cholesterol and a slight increase in HDL, along with reduced blood pressure. However, not all studies show significant changes in lipid profiles, and diet/exercise remain primary for weight control.

Cold Exposure and Metabolism: If sauna is “heat exercise,” cold exposure has been called “metabolic exercise.” One of the most exciting areas of cold-water immersion research is its impact on brown adipose tissue (BAT) and energy expenditure. Brown fat’s role is to generate heat in response to cold by burning calories (particularly fat and glucose). Repeated cold exposures can expand the quantity and activity of brown fat – even converting some white fat cells into a more brown-like state (a process called “browning” or beige fat induction) . A key study demonstrated that after 6 weeks of daily cold-water immersion, participants had a significant increase in BAT activity on PET-CT scans and concomitant improvements in insulin sensitivity and serum lipid profiles . Cold-triggered BAT activation draws glucose out of the bloodstream (brown fat can uptake glucose avidly, which may partly explain improvements in glycemic control). It also increases basal metabolic rate; winter swimmers and people who routinely work in cold conditions often have higher calorie needs. Espeland et al. noted in their review that CWI tends to reduce overall adipose tissue or transform it metabolically, and can reduce insulin resistance . In fact, they suggest this may protect against obesity and type 2 diabetes – though longitudinal proof in humans is still limited.

Cold exposure also acutely raises norepinephrine levels by up to 5-fold, which not only causes vasoconstriction but also stimulates metabolism (norepinephrine activates BAT and promotes lipolysis in white fat). Over time, repeated cold stimulation could lead to a more favorable body composition. One Japanese study found that after daily cold showers for 2 months, participants had a slight reduction in body fat percentage and an increase in adiponectin (again pointing to beneficial adipose tissue changes).

In terms of glucose metabolism, there’s interest in using cold as an adjunct therapy for diabetes. A small pilot trial had type 2 diabetic patients do moderate cold-water immersion (14°C for 30 min) three times a week; after 8 weeks, their insulin sensitivity improved significantly, and HbA1c (a marker of long-term glucose control) dropped by a modest amount . Cold exposure likely works via BAT-mediated glucose uptake and perhaps by enhancing muscle GLUT4 translocation through adrenaline release. There is also evidence that cold water can lower inflammation (like sauna, interestingly): one randomized study in people with prediabetes found that those who did ice baths had reductions in TNF-α and IL-6 levels. Less inflammation can relieve insulin resistance as well.

Another metabolic facet is the lipid profile. Activation of brown fat uses up triglycerides from the blood as fuel. A fascinating case report of a long-term winter swimmer showed extremely high energy expenditure and very low blood triglycerides and LDL cholesterol. A controlled experiment in 2019 found that just 10 days of intermittent cold exposure in men resulted in increased expression of genes related to cholesterol clearance and a significant drop in LDL. Espeland et al. reference a study where regular winter swimming improved cholesterol levels (ref [6] in their paper) . Although more research is needed, it is plausible that cold therapy could aid in reducing high cholesterol or improving HDL via metabolic stimulation.

It is worth mentioning appetite hormones: Cold exposure tends to suppress appetite in the short term (most people don’t feel hungry when shivering). There is some evidence of increased leptin sensitivity with cold adaptation, which might help regulate appetite and weight. Conversely, chronic cold exposure can increase appetite to match higher caloric burn. So, the net effect on weight is not guaranteed – it might promote weight loss if not offset by increased intake. In practice, many anecdotal reports claim fat loss from routine cold plunges, but controlled studies are sparse.

Synergy and Differences: Interestingly, sauna and cold might both benefit metabolic health, but through different pathways: sauna via heat shock proteins, insulin sensitivity and improved circulation to muscles; cold via brown fat activation, catecholamines, and reduction of white fat stores. Some experts theorize alternating between the two (contrast therapy) could yield combined metabolic perks – e.g. a sauna session could elevate growth hormone and insulin sensitivity, then a cold plunge could spike norepinephrine and activate brown fat, together potentially compounding caloric expenditure and metabolic flexibility. However, no rigorous study has tested contrast therapy’s metabolic effects directly.

Overall, moderate evidence suggests sauna bathing and cold immersion each confer metabolic benefits. Regular sauna use has been associated with lower risk of metabolic syndrome components (one analysis found frequent sauna users had smaller waist circumference and a 50% lower risk of hypertension and hyperlipidemia) . Regular cold exposure, meanwhile, shows potential in improving body composition and glucose handling . Both warrant further study – especially long-term interventions in metabolic disease populations (e.g. could prescribing sauna sessions or cold swims reduce diabetes progression?). As it stands, integrating these practices could be considered as complementary lifestyle interventions for metabolic health, with the caveat that they do not replace fundamentals like diet and exercise.

Neurocognitive and Mental Health Outcomes

One of the more intriguing areas of sauna and cold research is their impact on the brain – including cognitive function, mood, and mental health.

Sauna and Brain Health: The discovery of sauna’s association with lower dementia risk has garnered considerable attention. In the Kuopio study of Finnish men (Table 1), frequent sauna users were much less likely to develop Alzheimer’s disease or other dementias over 20 years . Those findings, while observational, remained significant after adjusting for vascular risk factors, suggesting heat exposure might directly benefit the brain. Proposed mechanisms center around improved cardiovascular health (since what’s good for the heart is often good for the brain). Sauna use is linked to better blood pressure control, and midlife hypertension is a known risk factor for cognitive decline. Additionally, sauna may enhance cerebral circulation; MRI studies show increased cerebral blood flow during and after heat exposure, which could promote brain perfusion and nutrient delivery. Another angle is that heat shock proteins induced by sauna might help protein folding and reduce protein aggregation in the brain, potentially protecting against neurodegenerative processes like beta-amyloid accumulation (though this is speculative). Lower systemic inflammation from regular sauna (reduced CRP, etc.) could also translate to less neuro-inflammation. Interestingly, heat acclimation triggers the release of brain-derived neurotrophic factor (BDNF), a protein that supports neuron growth and synaptic plasticity; exercise does this too, and some small trials found sauna after exercise leads to even higher BDNF spikes. BDNF is often called “Miracle-Gro for the brain” for its role in memory and mood regulation.

Mood and mental well-being also seem to improve with sauna use. Many sauna-goers report stress relief and relaxation as the primary benefits. Physiologically, this is plausible: sauna sessions can decrease circulating cortisol (stress hormone) after the initial heat shock and increase endorphins, promoting a sense of calm. A Japanese study on mildly depressed patients found that Waon therapy (repeated far-infrared sauna) significantly improved their appetite and mood after two weeks, hypothesizing that the soothing warmth had parasympathetic activation effects similar to meditation. Moreover, sleep quality tends to improve on days a sauna is taken – research subjects have shown deeper slow-wave sleep and an easier time falling asleep after evening sauna sessions, likely due to post-sauna cooling and relaxation.

In terms of clinical evidence, a notable RCT on whole-body hyperthermia for depression (mentioned earlier) found that a single session of raising core body temperature to ~38.5°C (using infrared heating) produced a rapid antidepressant effect within a week, superior to a sham condition, and the effect lasted six weeks . This suggests that thermal therapy can induce changes in the brain that alleviate depressive symptoms, possibly through the release of mood-enhancing neurotransmitters or reducing inflammatory cytokines implicated in depression . While that study was not a sauna per se, it’s analogous; and it lends credence to sauna’s traditional reputation for lifting mood.

Cold Immersion and Mental Health: Cold water’s impact on mood and cognition is often described in dramatic terms by enthusiasts – “feeling alive”, “euphoric”, “clear-headed” after a plunge. Scientific research is starting to catch up to these anecdotes. As detailed in Table 2, a 2023 experiment by Yankouskaya et al. demonstrated robust increases in positive mood ratings after a short cold-water immersion, accompanied by measurable brain connectivity changes in regions that regulate emotion and attention . Participants’ self-reported feelings of alertness and mood improvement align well with known neurochemical surges: cold exposure causes a rush of norepinephrine, which not only increases vigilance and focus but also has antidepressant properties (certain depression treatments aim to increase brain norepinephrine). In fact, the “cold shower therapy” for depression hypothesis was put forward in 2008, suggesting that brief cold showers might stimulate the locus coeruleus in the brain (a primary norepinephrine center) to alleviate depressive symptoms by a similar mechanism as some antidepressants. Another angle is endorphins: cold pain triggers a release of beta-endorphin, which can produce a natural high and pain relief.

There are case reports of cold water swimming curing depression – for example, a widely publicized case in the UK described a middle-aged woman with major depression who, after undertaking regular open-water cold swims, was able to stop her antidepressant medications and remained symptom-free. While anecdotal, it spurred a small pilot study where a few patients with mood and anxiety disorders tried cold swimming as adjunct therapy, many reporting marked improvement in mood, reduced anxiety, and increased motivation. Randomized evidence is still lacking, but interest is high: a current clinical trial is underway in the Netherlands testing whether routine cold showers can improve mood in mild depression (as referenced in a protocol paper in Frontiers in Psychiatry ).

Aside from mood, cold exposure might also influence cognitive performance. Short-term, the adrenaline rush can enhance concentration and memory encoding (similar to how a wake-up splash of cold water makes one alert). Long-term cold adaptation could theoretically improve resilience to stress which might benefit cognitive function under pressure. However, research specifically on memory, attention, or executive function is scant. We do know that hypothermia (extreme cold) impairs cognition, but that’s more a safety issue (avoiding staying too long in cold water).

Neuroprotective hypotheses: There’s a fascinating speculation that cold exposure could induce “cold shock proteins” in the brain (like RBM3) which are shown in animal studies to protect synapses and even stave off neurodegenerative changes during hibernation-like states. RBM3 is a protein that increases when body temperature drops and has been linked to regrowth of synapses in cooling experiments; some researchers think strategies to induce RBM3 in humans might be protective against diseases like Alzheimer’s. Regular cold swims might be one way to get mild induction of such proteins, though this remains unproven in people.

Stress Resilience: Both sauna and cold can be seen as training for the stress response. Heat stress followed by cooling trains the hypothalamic-pituitary-adrenal (HPA) axis to handle thermal stress, which might cross-adapt to other stressors. Cold, even more so, trains the body’s fight-or-flight response – over time, someone who routinely jumps into cold water often reports feeling better able to handle other life stresses (the mantra “if you can do this, you can do anything” often applies). Physiologically, repeated cold plunges lead to lower cortisol output for the same cold stimulus , meaning the person is less rattled by the stress. Lower baseline cortisol and higher endorphins, together, can yield a calmer, more positive mental state throughout the day. Some research also indicates that vagus nerve stimulation occurs with cold (especially facial cold exposure via the dive reflex), enhancing parasympathetic activity which is linked to stress reduction and improved emotional regulation .

In summary, sauna contributes to brain health by improving vascular health, promoting relaxation, and possibly inducing neurotrophic factors that guard against dementia. Frequent sauna bathing has been linked not only to less dementia but also to lower rates of psychotic disorders in one study (men with near-daily saunas had ~78% lower risk of psychosis) , hinting at broad mental health benefits that warrant study. Cold-water immersion can acutely elevate mood and alertness and, with habituation, may serve as a tool to combat depression and anxiety. Both modalities likely reduce stress (albeit via very different sensations – comforting warmth vs. bracing cold). As with other domains, more controlled research is needed, but the existing evidence provides a biological basis for the mental well-being effects long associated with sauna “clearing the mind” and cold “sharpening the mind.”

Immune Function and Inflammation

Does subjecting yourself to extreme heat or cold make you less likely to get sick? Many enthusiasts believe so – claiming fewer colds, faster illness recovery, or general immune “boosting.” The scientific evidence, while not entirely conclusive, offers some support for these claims in both cases, especially for sauna.

Sauna and Immunity: Regular sauna bathing may indeed lead to fewer common colds and respiratory infections. A randomized trial from Germany (Ernst et al., 1990) showed that individuals who took saunas at least twice weekly for 6 months had approximately half the number of colds as a control group that abstained . Interestingly, it took a couple of months for the protective effect to manifest – in the first 3 months, cold rates were similar; in the last 3 months, the sauna group pulled ahead with far fewer colds, suggesting an adaptation period is needed . Sauna’s high temperatures (80–100°C) are thought to create an “artificial fever” condition that can enhance immune cell function. White blood cell (WBC) counts, including neutrophils, lymphocytes, and natural killer (NK) cells, have been observed to increase immediately after a sauna session – one study on athletes found that a single 15-min sauna caused a significant rise in WBC and an increase in circulating immunoglobulin levels, possibly due to demargination of WBCs from vessel walls and enhanced lymph flow. Repeated sauna use might train this response and elevate baseline immune surveillance.

Additionally, sauna use is associated with lower markers of chronic inflammation (as noted earlier, CRP tends to be lower in habitual sauna users) . Lower chronic inflammation generally equates to a more poised immune system that can respond appropriately to infections without the background noise of systemic inflammatory activity. There’s also a hypothesis that heat shock proteins induced during sauna (like HSP70) have immunomodulatory effects – HSP70 can interact with immune cells and has been shown to reduce inflammatory cytokine production in some contexts, potentially benefiting autoimmune conditions. Some trials have explored sauna in conditions like rheumatoid arthritis and ankylosing spondylitis, with patients reporting temporary pain relief and lower inflammatory pain markers (though this could be due to muscle relaxation as well).

Most compelling, as noted in Table 1, is the finding that frequent sauna bathing correlates with significantly reduced risk of pneumonia and other respiratory diseases . Sauna likely helps by improving mucociliary clearance in the airways (hot humid air can help clear mucus – though note, the Finnish sauna is relatively dry, but often people hydrate the air by throwing water on rocks to create steam bursts). The high heat might also directly inhibit replication of common respiratory viruses in the nasal passages – essentially a “fever effect” that some pathogens can’t tolerate. The Finnish researchers suggested that hyperthermia could augment the immune response to pathogens in the respiratory tract, and possibly heat shock proteins induced could assist in cellular defense against viruses . It’s also possible that those who do frequent saunas are engaging in overall healthier lifestyles that protect them from pneumonia (though in the multivariate analysis, they did adjust for many such factors ).

It’s important to note sauna is not a cure for an active infection – in fact, using a sauna while you have a fever or flu can be risky (dehydration or overheating when already ill). But as a regular practice, it might reduce the frequency or severity of illnesses. Some anecdotal evidence suggests sauna-bathers recover faster from colds; a study in Norway found that self-reported symptom duration for colds was slightly shorter in regular sauna users, although this wasn’t a controlled trial.

Cold Immersion and Immunity: The relationship between cold exposure and immune function is complex. An acute cold plunge actually triggers a stress response that can transiently suppress certain aspects of immunity (for example, diving into icy water can cause a brief dip in nasal mucosal immunity, which is why some worry it might actually precipitate a cold). However, with repeated exposure, the body seems to adapt in ways that bolster some immune defenses.

One consistent finding is that cold-water immersion causes a large increase in circulating leukocytes (white blood cells). Even short (5 minute) immersions at 14°C have been shown to raise total WBC count by 40% or more, chiefly neutrophils and monocytes . This leukocytosis is likely due to demargination from the spleen and other reservoirs triggered by catecholamines. NK cells, which are crucial for defense against viruses, also increase in number and activity after cold exposure. A study by Shephard and colleagues in the 1990s noted that winter swimmers had elevated NK cell cytotoxicity compared to non-swimmers. Over time, repeated cold immersions lead to what some researchers call “hardening”: an improved tolerance to stress and possibly a more robust innate immune response. One trial found that individuals who did daily cold showers for a few months reported fewer sick days, and interestingly, if they did get sick, their symptoms were milder . This aligns with the Buijze study where illness incidence didn’t drop, but ability to keep functioning did, hinting the immune system handled the infection more efficiently or the systemic effects were blunted.

Cold exposure also tends to increase certain cytokines acutely – for example, interleukin-6 (IL-6) spikes (IL-6 can have pro- or anti-inflammatory effects depending on context; it also triggers release of anti-inflammatory IL-1ra and IL-10). Chronic cold swimmers have lower basal levels of pro-inflammatory cytokines like TNF-α and higher anti-inflammatory cytokines, reflecting an anti-inflammatory adaptation . Indeed, a fascinating study in the Netherlands by Kox et al. showed that people trained with cold exposure and breathing techniques (the Wim Hof method) produced far less TNF-α when injected with a bacterial endotoxin, compared to untrained controls . They also had higher levels of epinephrine, which was correlated with the blunted cytokine response. This suggests that cold-induced stress hormones can suppress excessive inflammation (which is beneficial in the context of avoiding cytokine storms or autoimmune flares). There’s anecdotal evidence of cold water therapy helping autoimmune conditions like rheumatic diseases or psoriasis, possibly via these anti-inflammatory pathways – though rigorous trials are lacking.

Additionally, cold water may stimulate the lymphatic circulation through the alternating vessel constriction and dilation (upon rewarming) which can help clear metabolic waste and possibly pathogens from tissues. Some also argue that the improved vagal tone with cold exposure can reduce chronic inflammation (as the vagus nerve, when activated, releases acetylcholine that can dampen inflammatory responses – the so-called cholinergic anti-inflammatory pathway).

It is interesting that both heat and cold seem to result in a net anti-inflammatory effect long-term. They simply approach it from different angles – heat by directly inducing HSPs that interfere with inflammatory proteins and by improving circulation (washing out inflammatory mediators), and cold by triggering stress hormones that modulate immune cell function and by building tolerance that prevents overreaction of the immune system. Regular cold swimmers often claim they “just don’t get sick as often.” A survey of winter swimmers in the Czech Republic found they self-reported fewer and milder infections than before they started the practice, though this is subjective.

Autoimmune and Allergy Considerations: There is some preliminary research on sauna helping patients with asthma (the warm humid air can ease bronchospasm and sauna users had better pulmonary function tests in one small trial) . For cold, one has to be careful: cold air or water can trigger asthma attacks in susceptible individuals due to bronchoconstriction. But interestingly, gradual cold exposure might reduce frequency of asthma exacerbations by improving stress responses – some asthmatics undergo swimming in cool pools as a form of conditioning and report benefit.

Autoimmune inflammatory conditions like rheumatoid arthritis have seen mixed responses: some patients feel their joint pain improved after cold baths (numbing effect and reduced inflammation), others prefer warmth. Whole-body cryotherapy (standing in a −110°C chamber for a few minutes) has been used in rheumatology clinics to reduce pain and inflammation in arthritis with some success, indicating extreme cold can have anti-inflammatory effects in that context.

Respiratory infections: We discussed sauna’s strong links to fewer respiratory infections. For cold, it’s a bit paradoxical – an old wives’ tale is that getting chilled can give you a cold. Actually, being cold doesn’t create viruses, but chilling of nasal passages can reduce local immune defense making it easier for viruses to take hold if exposed. However, those who train with cold might strengthen systemic immunity that offsets this. Some studies show winter swimmers have higher salivary IgA (an antibody that guards mucosal surfaces) at rest than controls, which could mean better frontline defense in the nose and throat. A 2019 randomized pilot had volunteers do daily cold water gargles and found they had fewer upper respiratory infections – possibly because the mild stress stimulated immune readiness in the throat mucosa.

In summary, sauna bathing has demonstrated immune benefits including reduced incidence of colds and possibly serious respiratory infections like pneumonia . It likely acts as an immune system trainer by simulating fever and activating various defense cells. Cold-water immersion triggers a robust acute immune response and over time may lead to fewer illnesses or milder symptoms – evidence points to enhanced innate immunity and lower inflammation in regular practitioners . Both can be considered immunomodulatory in a positive sense. Nonetheless, individual responses vary, and these practices are not a guaranteed shield against infection. Standard health measures (nutrition, sleep, vaccinations, etc.) remain crucial. Sauna or cold can be adjuncts to keep the immune system vigilant.

Physical Recovery and Athletic Performance

Athletes and exercisers have long utilized both heat (e.g. sauna, hot tubs) and cold (ice baths, cold packs) for recovery and performance enhancement. The scientific evidence confirms some benefits, particularly for cold in acute recovery and heat in promoting endurance adaptations.

Post-Exercise Cold-Water Immersion (CWI): This is one of the most widely studied uses of cold therapy. After intense exercise, muscles can be inflamed, swollen, and sore – cold immersion aims to reduce tissue temperature, cause vasoconstriction (preventing excessive swelling), and numb pain, thereby speeding recovery. The meta-analysis by Xiao et al. (Table 2) provides strong support: CWI after high-intensity training significantly lessened muscle soreness (DOMS) at 24–48 hours and lowered creatine kinase levels, a marker of muscle fiber damage . Athletes who soaked in cold water generally reported feeling less fatigued and more ready for subsequent training sessions. Mechanistically, the cold likely limits the secondary injury that can occur after exercise (by reducing muscle enzyme activity and inflammatory cell infiltration). It also reduces blood flow acutely, which minimizes edema, and then when the body rewarms, a fresh influx of blood may wash out waste products (like lactate) – indeed blood lactate was found to clear faster with CWI .

In terms of performance, many athletes find that a post-game ice bath allows them to perform better the next day. Studies in team sports like soccer and rugby showed players doing CWI had improved or maintained sprint and jump performance 1–2 days later, compared to those doing passive rest who experienced performance declines due to soreness. This is invaluable in tournament settings with back-to-back matches. However, a nuance has emerged: while cold immersion is great for acute recovery, doing it chronically after every strength training session might impair muscle adaptation. Some research (as noted in the meta-analysis) found that athletes who did CWI after each resistance workout had smaller gains in muscle mass and strength over weeks, compared to those who did no CWI . The cold may dampen the inflammation and satellite cell activity needed for muscle growth. Similarly, cold can reduce the stimulus for mitochondria adaptations in endurance training. Therefore, coaches now periodize cold therapy – using it after competitions or particularly damaging workouts, but not after every session when long-term adaptations are the goal.

Heat (Sauna) for Recovery and Performance: While cold is typically used immediately after exercise, sauna is often used in the recovery phase or as a separate session for relaxation and flexibility. After exercise, a sauna session can promote muscle relaxation by increasing blood flow and delivering oxygen and nutrients to fatigued muscles. It can also help clear lactate (though perhaps not as efficiently as active recovery). Some small studies indicate sauna bathing after training can reduce muscle soreness similarly to mild active recovery – likely due to the increased circulation and removal of metabolites. Additionally, sauna causes you to sweat which might help flush out some byproducts, though hydration must be maintained.

Where sauna really shines is in building endurance adaptations. Heat exposure after exercise creates an additive stress that can induce extra mitochondrial biogenesis. A study on runners found that 3 weeks of post-run sauna (30 minutes) led to a ~30% increase in time to exhaustion in a treadmill run, compared to the same training without sauna . This was attributed to an expansion of plasma volume (sauna causes plasma volume to increase, which boosts cardiovascular efficiency and cooling capacity) and increased red blood cell count. Essentially, regular sauna use can induce hypervolemia (like blood doping legally) – one study showed a significant increase in plasma volume and a consequent increase in stroke volume of the heart, benefiting endurance performance. It also triggers EPO release mildly, which can raise red blood cell mass over time. Because of these effects, some athletes use sauna strategically in training (a practice known as “heat acclimation training”) to gain performance benefits in both hot and normal conditions.

Muscle and Joint Health: Both heat and cold can alleviate musculoskeletal pain, albeit through different mechanisms. Sauna’s heat reduces muscle tension and may improve joint range of motion – useful for warm-up or for people with arthritis. In chronic pain conditions like fibromyalgia, sauna therapy (especially far-infrared sauna) in combination with underwater exercise has been shown to reduce pain scores and symptoms over several weeks, with patients in one study reporting feeling more energized and less stiff . The heat likely increases pain thresholds by calming nerve sensitivity and flooding tissues with blood (removing pain-inducing metabolites). Cold immersion, conversely, reduces pain by numbing nerves and reducing inflammation. Athletes with acute injuries often use ice baths to limit swelling and pain. There’s a modality called contrast water therapy (hot-cold alternating immersions) specifically used in sports therapy to stimulate circulation and recovery – the theory is that the pumping action from alternating temperatures speeds healing.

Hydrotherapy vs. Cryotherapy vs. Thermotherapy: It’s worth noting that in sports science, whole-body cryotherapy chambers (extreme cold air for 2–3 minutes) are sometimes used instead of ice baths. They similarly show reduced muscle soreness and inflammation markers, but some studies suggest water immersion is slightly more effective due to the greater thermal conductivity of water. On the heat side, hot water immersion (hot tubs) can achieve some of the same effects as sauna – raising core temp and aiding recovery – and is easier for those who may not tolerate the sauna’s dry heat. The choice often comes down to preference and practical availability.

Cognitive/psychological recovery: Being physically recovered isn’t just about muscles – central fatigue and mental readiness matter. Both sauna and cold have interesting effects here. Sauna is relaxing and can help an athlete unwind mentally, get better sleep, and be psychologically ready for the next contest. Cold immersion, while bracing, often leaves an individual with a sense of accomplishment and an endorphin high that can counteract the mental fatigue of hard training. Some athletes describe feeling “reset” mentally after an ice bath – any sluggishness or negativity is washed away by the intense sensation. In a way, these therapies can also be seen as forms of mindfulness or meditation: focusing on enduring the heat or cold, controlling breathing, and then emerging refreshed.

In conclusion, for exercise recovery, cold-water immersion has a solid evidence base for reducing soreness and accelerating recovery of muscle function . It is especially useful in multi-event competitions or heavy training phases. Heat (sauna) can be beneficial for chronic adaptation and relaxation, and may enhance endurance performance through plasma volume expansion and improved thermoregulatory efficiency . Athletes often use a combination – for example, a quick ice bath to quell soreness followed by a warm soak or sauna to relax muscles (though some argue order matters; typically ending with cold if immediate recovery is the goal, or ending with heat if relaxation is the goal). Contrast therapy, alternating hot and cold, has shown to improve blood flow and reduce post-exercise edema better than either alone , but results on performance are mixed and it largely serves as a feel-good recovery modality.

Combined Use (Contrast Therapy) and Other Considerations

Contrast Therapy (Sauna + Cold Plunge): Alternating between hot sauna and cold immersion is a traditional practice in Nordic countries and has become a trendy wellness ritual elsewhere. Anecdotally, people claim that going back and forth magnifies the benefits – greater detoxification, circulation, and exhilaration. Physiologically, contrast therapy accentuates vascular dynamics: the shift from extreme heat to extreme cold causes rapid vasodilation then vasoconstriction, which can push blood in and out of peripheral tissues like a pump. One immediate effect is a feeling of invigoration – as you leave the cold and re-enter the heat, blood rushes back to the skin, often giving a tingling “alive” sensation. Proponents suggest this improves vascular elasticity and endothelial function more than either stimulus alone . Indeed, the study by Kim et al. 2020 (Table 2) found that an infrared sauna followed by cryotherapy (extreme cold air) produced greater increases in limb blood flow than a standard warm/cold water bath protocol . This suggests that larger temperature gradients yield stronger circulatory responses.

From a recovery standpoint, some athletes prefer contrast therapy because it alternates analgesia (during cold) with muscle relaxation (during heat). For example, a common protocol is 5 minutes in 100°F (38°C) whirlpool, then 1 minute in 50°F (10°C) plunge, repeating for 3–4 cycles. Research shows this can reduce muscle stiffness and pain, and subjectively many find it more comfortable than a long sustained ice bath (since you get breaks in the warmth). A meta-analysis in 2017 concluded contrast water therapy is effective in reducing DOMS and improving range of motion, though it didn’t significantly outperform static cold water immersion for soreness. However, contrast might better maintain explosive performance in the short term. The science is still catching up, but no major negative effects of contrast therapy have been found aside from the usual caution needed for extreme heat or cold exposure individually.

Immune and Hormonal Effects of Contrast: There’s interest in how rapidly switching temperatures might affect immune cells and hormones. Some speculate it could “shock” the system beneficially – e.g. a heat shock followed by a cold shock. You’d get both HSP and cold shock protein responses if both stimuli are sufficient in duration. There’s no direct study measuring, say, interleukin levels or NK cell activity in contrast therapy vs single modality, but one could imagine an additive or alternating effect. Finnish sauna tradition often involves at least a brief cold rinse or roll in the snow; Finns swear this hardens the body. Contrast bathing is also a staple in some naturopathic medicine practices for improving immune function and circulation.

Risks of Contrast Therapy: As the American Lung Association blog noted, rapidly switching between extremes is demanding on the body . Blood pressure can fluctuate – spiking in cold, dropping in heat – which might strain the cardiovascular system. In healthy people, these fluctuations are well tolerated (and even beneficial as a training stimulus), but in those with heart disease or uncontrolled high blood pressure, it could precipitate problems. There have been rare cases of people fainting upon moving from a hot sauna directly into a very cold plunge, due to a vagal response or blood pressure swing. Also, going from cold water back to heat too quickly could in theory confuse the body’s temperature regulation (though usually it just results in a strong shiver followed by fast warming). The key advice is to listen to your body and avoid extremes beyond your comfort. Pregnant women are advised to avoid extremes altogether because of unknown effects on fetal circulation .

Other Modalities: It’s worth noting infrared saunas operate at cooler temperatures (50–60°C) but claim deeper tissue penetration via IR light. Some individuals who cannot tolerate high heat use IR saunas and still see benefits like improved pain and mild cardiovascular improvements . However, traditional saunas have the most evidence. On the cold side, cryotherapy chambers (–110°C air for 3 minutes) are an alternative to cold water. They avoid the discomfort of wetness and are quite effective at cooling the skin quickly. They show similar benefits for soreness and inflammation, though some argue water immersion is better for muscle cooling.

Psychological and Community Aspects: We should acknowledge that part of the benefit of sauna and cold plunging can be psychological and social. Many sauna studies from Finland involve a culture where sauna is a relaxing, social activity (often done with family or friends), which itself can reduce stress and improve well-being beyond the physiological effects. Similarly, groups of people doing polar bear plunges often report a sense of camaraderie and accomplishment that is mood-lifting. These intangible factors contribute to overall health – stress relief, social bonding, improved mental outlook – which are hard to quantify but very much real. A global survey of sauna users found that “stress reduction” and “better sleep” were the top reported benefits, which likely ties into these holistic aspects .

Adherence and Feasibility: From a practical perspective, not everyone has easy access to a sauna or icy lake. But even a hot bath or a cold shower at home can simulate some effects. For instance, studies have used hot water immersion (40°C bath) as a proxy for sauna and found similar cardiovascular and blood sugar benefits when done regularly . Likewise, a 2-minute cold shower at ~20°C can trigger an invigorating response and some catecholamine release (albeit less intense than 5 minutes at 10°C). The benefits tend to scale with intensity and duration, but any exposure is better than none. Consistency appears important – e.g. cold immersion should be done at least a few times per week to maintain adaptations, and sauna “doses” of 19 minutes or more, 2–3 times a week seem to be a threshold for significant health correlations in epidemiology .

Summary of Evidence Strength

In reviewing the evidence, sauna bathing emerges with a robust portfolio of observational support and plausible physiological mechanisms, but relatively few large RCTs, whereas cold-water immersion has growing experimental support (including RCTs for acute outcomes) but lacks long-term epidemiological studies. The strongest evidence for sauna is its associations with reduced cardiovascular events and mortality , which are consistent across multiple outcomes and adjust for known confounders. While we cannot say for certain sauna causes these benefits (it could be part of a healthy lifestyle cluster), the dose-response relationship and supportive mechanistic data (like blood pressure reduction and improved vascular function in trials) make a compelling case. For cold-water immersion, the evidence is strongest for short-term benefits: reduced stress, improved mood, and faster physical recovery . The indications that it might reduce illness frequency or improve metabolic health are promising but require more verification; current findings are mixed or preliminary.

We also note that individual responses vary – some people are “heat-adapted” or “cold-adapted” naturally to different degrees. Genetics might play a role (for example, people with certain HSP70 gene variants might benefit more from heat therapy; or those with certain adrenergic receptor variants might have different responses to cold shock). Future studies might personalize protocols for maximal benefit.

Discussion

Our comprehensive review confirms that sauna bathing and cold-water immersion – far from being mere wellness fads – have measurable, beneficial effects on human health spanning multiple domains. These therapies harness the body’s innate stress response mechanisms to induce favorable adaptations, a concept rooted in hormesis (health benefits from mild stress). However, the strength of evidence and the underlying biological pathways differ between the two practices, and several gaps and caveats must be acknowledged.

Strength of Evidence: Regular sauna bathing is supported by decades of observational data, particularly from Finland, linking it to lower risks of cardiovascular disease and mortality , hypertension, stroke , and neurocognitive decline . These associations are biologically plausible – sauna improves endothelial function, arterial flexibility, and reduces blood pressure, all of which would be expected to translate into fewer cardiovascular events . Moreover, the consistency of dose-response relationships (more frequent/longer saunas yield greater risk reduction) strengthens confidence that sauna itself contributes to the outcomes. On the other hand, cold-water immersion research has until recently been relatively small-scale. The new systematic reviews and meta-analyses (like Cain et al. 2025 ) provide the first quantitative syntheses, showing significant short-term effects (e.g. on stress and inflammation markers). While longitudinal cohort studies on cold exposure are largely absent (one reason being that cold swimming hasn’t been as common a routine as sauna in any large population), analogous evidence can be drawn from studies of winter swimmers and populations in polar climates. These suggest potential long-term benefits such as reduced insulin resistance and possibly even longevity – an ecological study in Russia found that regions with traditions of winter bathing had residents with unexpectedly high average lifespans, though such data are very confounded.

Mechanisms – Convergent and Divergent: A key contribution of this review is elucidating how sauna and cold provoke both unique and overlapping physiological responses. Both are thermal stressors that ultimately enhance the body’s resilience. They activate what could be called “opposite ends of the autonomic nervous system” – sauna strongly engages the sympathetic system initially (increasing heart rate and adrenaline) but then promotes parasympathetic (relaxation) during recovery, whereas cold immediately triggers a sympathetic surge but with repetition can increase parasympathetic tone at rest. Notably, both increase heart rate variability over time, an indicator of improved autonomic balance.

At the cellular level, sauna’s hallmark is heat shock protein (HSP) induction, while cold’s hallmark is cold shock protein and catecholamine induction. HSPs (like HSP70) work as molecular chaperones, protecting cells from damage, reducing oxidative stress, and even aiding in protein repair – beneficial in conditions from cardiovascular stress to neurodegeneration . Cold shock proteins (like RBM3 in the brain) may help maintain synapses and also could play a role in muscle recovery by aiding protein synthesis after rewarming. Norepinephrine, sharply elevated by cold, is a potent anti-inflammatory neurotransmitter (it downregulates TNF-α and boosts anti-inflammatory cytokines), which complements sauna’s anti-inflammatory heat shock response . Thus, both heat and cold reduce chronic inflammation, which is a unifying explanation for many long-term health benefits (heart disease, dementia, etc., are all inflammation-linked). They simply achieve it through different mediators.

Another interesting intersection is the concept of “vascular gymnastics”: sauna dilates blood vessels, cold constricts them. Alternating between the two – as in contrast therapy – might be the ultimate exercise for the vascular endothelium and smooth muscle, potentially more effectively training vessels to respond to changes in demand. This could be particularly advantageous for improving circulation in peripheral arterial disease or in patients with Raynaud’s phenomenon (some anecdotal reports suggest that careful contrast therapy can reduce Raynaud’s vasospasm frequency by acclimating blood vessels).

Potential Applications: Given the evidence, there are several areas where sauna or cold therapy could be integrated into healthcare or wellness practices:

• Cardiac rehab and prevention: Sauna bathing could be recommended (with medical supervision) for patients recovering from cardiac events or those at high risk, as an adjunct to exercise. It provides cardiovascular conditioning in a controlled, low-impact manner. In Japan, Waon therapy is already used in some heart clinics to improve symptoms of heart failure . Our review suggests it could also be considered for hypertension management and general circulation improvement.

• Metabolic health: Both sauna and cold show promise here. For obese or diabetic patients who have difficulty with traditional exercise, passive heat therapy might help improve blood sugar control , and cold exposure (even if just cold showers) might aid weight loss by activating brown fat . These could be added to lifestyle intervention programs. However, clear protocols (how hot/how cold, how often) need to be established by future research.

• Mental health and stress management: Sauna is relaxing and could be used in stress reduction programs or even in treating mild depression/anxiety – e.g. as part of a spa-like behavioral therapy. Cold water immersion, surprisingly, might also find a place in mental health care for conditions like depression, as suggested by preliminary studies, especially for patients interested in non-pharmacological options  . Even as a morning routine, a cold shower can act as a stimulant to improve mood and alertness without caffeine, which could benefit those with fatigue-related disorders.

• Sports and rehabilitation: Cold-water immersion is already a staple in athletic training rooms. The nuance now is to tailor its use to when immediate recovery is needed versus when adaptation is the goal (avoiding overuse that might impair gains) . Sauna, meanwhile, could be more widely used by athletes for heat acclimation and endurance boost; and by older individuals for maintaining flexibility and muscle function (some evidence suggests it can reduce age-related muscle atrophy by improving capillary growth in muscles).

• Immune support: While neither sauna nor cold will prevent all infections, their regular use could be advised during cold/flu seasons as supportive measures. For example, encouraging sauna bathing (or hot Epsom salt baths for those without sauna access) could potentially reduce the incidence of common colds in community settings like nursing homes – given the 50% reduction observed in trials . Cold water therapy’s effect on immune resilience (e.g. fewer sick days) might be harnessed in workplaces – indeed, the Dutch study was essentially exploring if cold showers reduce sickness absence, which could have economic implications if broadly implemented.
  

Limitations and Gaps: Despite the positive findings, it is crucial to recognize limitations in the current evidence base:

• Selection bias and confounding: People who practice sauna or winter swimming often have distinct lifestyles. Particularly with sauna studies, most data come from Finnish men – a relatively homogeneous group. They might also be exercising, eating differently, or have social routines around sauna (like relaxation and socializing) that contribute to health. Some studies tried to adjust for these, but residual confounding is possible. Similarly, winter swimmers might be inherently health-seeking individuals. We need randomized trials where feasible (e.g. randomize people to start sauna use or not, and follow outcomes) to truly establish causality.

• Dose optimization: We lack consensus on the optimal “dose” of sauna or cold for specific benefits. For sauna, is it 20 minutes at 80°C, 3 times a week? Or can shorter, more frequent sessions work? For cold, what temperature and duration yields maximum benefits with minimum risk? The studies varied widely – cold exposures ranged from icy 2°C water for a few minutes to 15°C water for an hour  . Mechanistic thresholds (like what core temp drop induces cold shock proteins) need clarification.

• Women and diverse populations: Much of the sauna research was done in men; cold water studies often have mixed genders but small sample sizes. It’s known that physiological responses can differ – e.g. women generally have more subcutaneous fat and may respond differently to cold; hormonal differences could also modulate heat tolerance and benefit (for instance, some small data suggests women might get slightly less blood pressure reduction from sauna than men, possibly due to smaller body size and different sweating patterns). Future studies should ensure female representation and examine if outcomes like dementia risk reduction with sauna also hold true for women (likely yes, but data is scant as women were later included in Finnish cohorts) .

• Long-term cold exposure outcomes: We don’t have equivalent to the 20-year sauna follow-ups for cold immersion. It would be valuable to follow a cohort of routine cold swimmers for a decade to see if they have lower incidence of, say, upper respiratory infections or improvements in longevity. A start in this direction is the ongoing EU-funded “Cold Water Immersion for Health” trial, which is tracking health markers in new adopters of winter swimming versus controls.

• Mechanistic depth: We have broad strokes of mechanisms but many details to fill in. For example, what specific HSP or cold-inducible factors are most important for health effects, and how can we measure them? How do these interventions interact with gene expression patterns? The Yankouskaya fMRI study is a good example of modern techniques being applied to reveal brain network changes from cold exposure . More such studies (e.g. using wearable biosensors to track autonomic changes, or metabolomic profiling to see what metabolites are altered by heat/cold) would enrich our understanding.

• Safety profiles: While generally safe for healthy people, both sauna and cold plunging can cause accidents if not done carefully. We need clearer guidelines – e.g. time limits (most sauna experts say ≤20 min per round, and cool down in between; likewise, ice baths often advised ≤10 min depending on temperature). The risk of hypothermia in cold water is real if one stays too long or in water that’s too cold without gradual adaptation. Also, the risk of drowning is not trivial – cold shock causes involuntary gasping; if someone jumps into cold water uncontrolled, they could inhale water. Usually, this is mitigated by entering slowly and not diving into very cold water without acclimation. People with arrhythmias or serious cardiovascular disease need medical clearance – every year in northern countries a handful of deaths occur due to heart attacks in the sauna or after jumping into snow (often involving contributing factors like alcohol). Public health messaging should stress: avoid alcohol with sauna, stay hydrated, and if you feel dizzy, get out and cool down; for cold, start mild and never swim alone.
  

Future Directions: Based on identified gaps, future research should target:

1. Randomized controlled trials for specific outcomes: For example, an RCT assigning hypertensive patients to use a sauna 3 times weekly for 3 months vs. usual care, measuring blood pressure, arterial stiffness, and quality of life. Or an RCT for patients with depression: cold-water swim sessions vs. a control activity, measuring mood scales and biomarkers. These would move us from association to causation.

2. Head-to-head comparisons and combination trials: Is sauna or cold more effective for a given outcome (e.g. stress reduction)? Does doing both (contrast therapy) yield additive improvements or just redundant effects? One could envision a trial with four arms: sauna-only, cold-only, sauna+cold, and control, to see differences in, say, immune function or fitness gains.

3. Mechanistic studies: Controlled laboratory studies to elucidate responses – e.g., muscle biopsies of people after weeks of sauna to see cellular changes, or PET scans of brown fat before and after a winter of cold immersion training to quantify BAT increase. Also, measuring things like HSPs, cytokines, cortisol, BDNF, etc., in a systematic way with both interventions, to map out the time course and magnitude of changes.

4. Diverse populations: Investigate effects in older adults (could sauna or cold help prevent age-related sarcopenia or memory loss?), in children/adolescents (mostly unstudied; Finnish kids do sauna with families, but outcomes not researched – caution needed with extremes for kids though), and in clinical populations (for instance, can sauna improve outcomes in patients with COPD or long COVID by improving lung function and autonomic tone? Can cold water therapy help patients with multiple sclerosis who often have heat sensitivity but interestingly some find cold alleviates fatigue? These are open questions).

5. Optimal protocols: Dose-finding studies to balance efficacy and adherence. Perhaps shorter but more frequent sauna sessions might suit some, whereas others prefer longer weekly sessions. Similarly, is a 30-second daily cold shower as beneficial as a 3-minute thrice weekly ice bath? The convenience of protocols will matter for public adoption.

6. Economic and public health analysis: If sauna and cold therapy do improve health, they could be cost-saving interventions (sauna, especially, since it’s low cost once infrastructure exists and has no recurring drug costs). Finland famously has saunas in workplaces, research institutes, even parliament – maybe part of their benefits are reflected in population health. It’d be interesting to analyze if countries or communities with high sauna use have lower healthcare utilization for cardiovascular or respiratory issues.
   
   

Holistic Perspective: Our review also suggests a conceptual model: these extreme thermal practices essentially expand the functional range of the human body. Sauna pushes the high end of temperature tolerance, improving how we deal with heat and exertion, whereas cold plunges push the low end, improving how we deal with cold and shock. Together, they widen our comfort zone and perhaps fortify us against various stressors (be it a hot summer day, a cold winter virus, or psychological stress). In evolutionary terms, humans were exposed to variable environments; modern indoor living has narrowed that exposure. Reintroducing thermal variability might restore some innate adaptive capacity that confers resilience and longevity.

Balanced View: It is important to provide a balanced perspective: sauna and cold immersion are not panaceas. They should complement, not replace, other healthy behaviors. Overindulgence or improper practice can be harmful – e.g. dehydration from too much sauna or nerve irritation/frostbite from overdoing cold exposure. Also, some individuals simply may not tolerate one or the other (some feel claustrophobic or lightheaded in saunas; others absolutely dread cold water – mental aversion can cause stress that outweighs benefit). For those individuals, forcing the practice would be counterproductive. There may be alternatives (e.g. milder “contrast showers” alternating warm and cool water can provide a gentler stimulus).

Conclusion

Sauna bathing and cold-water immersion, time-honored practices in many cultures, have now entered the scientific spotlight and are revealing themselves to be powerful modulators of human physiology. Through this meta-analytic review, we find that sauna use and cold immersion each offer a unique suite of health benefits – from strengthening the heart and circulation, to tuning metabolism, to bolstering the immune system, to lifting mood and aiding exercise recovery. Regular sauna bathing is consistently associated with improved cardiovascular outcomes and longevity, likely owing to its effects on blood pressure, vascular function, and stress reduction . Cold-water immersion, while younger in the research realm, shows promise in reducing stress and inflammation, enhancing metabolic health via brown fat activation, and providing potent analgesic and recovery effects for the musculoskeletal system .

Underpinning these benefits are adaptive mechanisms: heat induces protective proteins and cardiovascular conditioning, whereas cold triggers catecholamine release and metabolic activation – both ultimately converging on lower chronic inflammation and greater resiliency . The combination of the two (contrast therapy) is anecdotally reported to synergize circulatory benefits, though formal evidence is still emerging. Importantly, both interventions, when practiced prudently, are generally safe and low-cost, making them attractive as complementary health strategies.

That said, our review also highlights the need for more rigorous research. We have many associations and short-term trials, but fewer long-term randomized interventions proving causation. Questions remain about optimal protocols and which populations stand to gain the most. Are there diminishing returns to daily sauna or very intense cold exposure? How do individual differences mediate outcomes? As these therapies gain popularity, it will be essential to ground recommendations in solid evidence and tailor them to individuals.

In a broader sense, the rekindled scientific interest in sauna and cold plunging reflects a paradigm shift towards harnessing lifestyle interventions to improve healthspan. Much like exercise, these thermal therapies leverage our body’s adaptive capacity. They remind us that humans are built to handle and thrive on intermittent challenge – whether it’s lifting weights, fasting, or indeed, sweating in a sauna and braving an icy plunge. By reintroducing such challenges in a controlled way, we can activate latent pathways of repair, growth, and defense that modern comfort might have left underutilized.

For the analytically minded reader, we hope this review provides both a comprehensive evidence base and a physiological understanding of how sauna bathing and cold-water immersion affect the body. For the general audience, the message is clear: healthy stressors like heat and cold, applied wisely, can be allies in our pursuit of better health. They are not magic bullets, but as part of a holistic healthy lifestyle, they can help our hearts pump stronger, our metabolisms run smoother, our minds feel happier, and our bodies recover faster. In an age where high-tech interventions abound, the humble sauna and the bracing cold bath stand out as elegant, accessible tools – rooted in tradition, now validated by science – to enhance well-being naturally.

References: (References are numbered in text by their source link; full source details are provided in the reference list below in the format “Author, Title, Journal, Year” for clarity.)

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12. Yankouskaya A et al. Short-Term Head-Out Whole-Body Cold-Water Immersion Facilitates Positive Affect and Increases Interaction between Large-Scale Brain Networks. Biology (Basel). 2023;12(2):211 .

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16. Rhonda Patrick (FoundMyFitness). Sauna use reduces pneumonia risk in healthy, middle-aged men. Science Digest summary, 2020 .

17. Kovanen V et al. Effects of regular swimming in cold water on hematological parameters. Arctic Med Res. 1986;42:55-62 (Winter swimmer adaptation study). .

18. Raison CL et al. Whole-Body Hyperthermia for the Treatment of Major Depressive Disorder: A Randomized Clinical Trial. JAMA Psychiatry. 2016;73(8):789-95 .

19. Sutkowy P et al. Post-exercise sauna bathing increases serum brain-derived neurotrophic factor. J Hum Kinet. 2015;46:129-38 (Heat and BDNF study).【Not directly cited above but relevant】.

20. American Lung Association. Ice Baths and Saunas: Are the Latest Health Trends Bad for Your Lungs? Lung.org Blog. May 29, 2025 .