Sauna Science
Shortcut to the relationships between sauna use and physical performance, cognition, and health.
Friendly disclaimer: I am not an expert or a licensed medical professional, so nothing in this post is medical advice in any way, shape, or form. You are responsible for conducting your own research, consulting your doctor, and making your own choices.
Cedar Serenity
Stepping past the door, a wave of immediate relief crashed down on me, as the stinging winter cold seemingly vanished. Seated on that warm, cedar bench, I knew that the sauna wouldn’t save me from the drills and sprints I would soon run on the maple hardwood next door, but it did serve as a useful procrastination tool. And, at the time, that’s all I envisioned the sauna as: a comforting checkpoint for hanging out and maybe relieving stress. But, though it can be cozy and relaxing, when utilized tactically, the sauna can also be a tool for challenging one’s cardiovascular and thermoregulatory systems, improving physical performance, cognition, and health as a result. In this Shortcut, we’ll walk through some of the interesting literature describing the benefits of sauna, from muscle preservation to mental health improvement and mortality reduction.
For a comprehensive deep dive on sauna research as a whole, check out Patrick and Johnson’s review here. Much of the information we’ll cover in this Shortcut traces back to that paper.
Physical Performance
Although simply sitting in a hot room for 10-30 minutes doesn’t seem like it would impact physical performance at all – let alone significantly – there is evidence pointing to the contrary. (I, II, III, IV, V) For example, Scoon et al. found that ~3 post-exercise sauna sessions per week for 3 weeks in ~89.9 °C (~193 °F) for ~30 minutes increased time to exhaustion by 32% relative to control. (IV) This magnitude of change translates to around a 1.9% improvement in race time, or about a 20 second faster 5k time for the runners in the study who averaged 17.5 minutes for their best 5k time at baseline. (IV)
“Relative to control, sauna bathing increased run time to exhaustion by 32% (90% confidence limits 21–43%), which is equivalent to an enhancement of ∼1.9% (1.3–2.4%) in an endurance time trial. Plasma and red-cell volumes increased by 7.1% (5.6–8.7%) and 3.5% (−0.8% to 8.1%) respectively, after sauna relative to control. Change in performance had high correlations with change in plasma volume (0.96, 0.76–0.99) and total blood volume (0.94, 0.66–0.99), but the correlation with change in red cell volume was unclear (0.48, −0.40 to 0.90).” (IV)
In addition to the sauna, non-traditional heat therapies, such as local heat application and sauna-suit training, have demonstrated effectiveness for improving physical performance. For instance, Hafen and colleagues’ experiment on applying pulsed shortwave diathermy – a localized heat therapy – daily for 2 hours to subjects’ thighs during a 10-day immobilization period revealed a ~38% relative reduction in whole muscle vastus lateralis atrophy and a 46% relative reduction in vastus lateralis muscle fiber atrophy – atrophy being measure by whole muscle and muscle fiber cross sectional area, respectively. (II)
“Vastus lateralis CSA decreased 7.3% with [immobilization alone]...and 4.5% with [immobilization plus daily pulsed shortwave diathermy] …Myofiber CSA decreased by 10.8% with [immobilization alone]...but only 5.8% with [immobilization plus daily pulsed shortwave diathermy].” (II)
Van de Velde et al.’s study on sauna suit training also presents intriguing results, demonstrating a 9.1% increase in VO2 max, 6.9% increase in ventilatory threshold, 4% decrease in 5k time in ~35° C (95 °F), and a 2.9% decrease in 5k time in ~18° C(~64 °F) following just 2 weeks of utilizing a sauna suit during training runs. (III) Impressively, in the experiment, the percentage increases in 5k time equated to 52 and 38 second improvements in hot and normal temperatures, respectively. (III) In addition, the subjects training in sauna suits exhibited lower body temperatures, increased sweat rates, and similar heart rates throughout their final 5k time trial as compared to their baseline trial, even though they were running at faster paces. (III)
“Repeated measures ANOVA showed that body core temperature responses…throughout the 5km heat time trial were significantly lower (p<0.05) following 2wk of training in the sauna suit treatment group…[and] that heart rate responses…throughout the 5km heat time trial were similar (p>0.05) after 2wk of training in the sauna suit. These similar heart rate responses existed despite the significantly faster 5km heat time trial performance post-training.” (III)
As a collective, this data on sauna and alternative heat therapies’ use cases for enhancing physical performance – specifically improving endurance and preserving muscle mass during immobilization – offers valuable insight for both athletes and individuals restricted from physical activity, such as the elderly and those recovering from injuries. (I, II, III, IV, V) And, though the sample sizes from these studies are not huge, the magnitudes of effects are noteworthy considering such short time frames.
Cognition
Contrary to its less than obvious effects on physical performance, sauna bathing’s impacts on mental health are somewhat intuitive, as we regularly seek out hot environments for relaxation. If you’ve ever spent the day lounging at the beach or kicked back in a hot tub to blow off steam (pun-intended) you understand what I mean. Interestingly, heat therapy not only offers this subjective stress-relief but confers objective neural benefits as well. (I, IX, X, XI) For example, increasing core body temperature – whether it be through exercise or other means – increases brain-derived neurotrophic factor (BNDF), a neural growth factor positively associated with neural plasticity, cognitive function, and neural health, and inversely associated with neurodegenerative disease. (I, IX) Kojima et al. found that 20-minute head-out, water immersion in 42.0 °C (107.6 °F) increased blood BDNF levels by 66% in ~25 year-old males. (I, IX)
“BDNF promotes and improves neuronal plasticity, suppresses neuronal cell death, induces neural regeneration and stimulates neuronal survival, particularly motor and sensory neurons of the peripheral and central nervous systems…Moreover, BDNF plays important roles in memory, learning, mood disorders, food intake and energy metabolism…Low blood levels of BDNF have been reported in patients with neurodegenerative diseases, including Alzheimer’s disease and major depression.” (IX)
Heat therapy has also demonstrated practical potential, such as positive results in acute and short-term treatment of major depressive disorder (MDD). (I, X, XI) In particular, infrared sauna-bathing, in which infrared rays directed towards subjects in a ~140 °F environment induce increases in core body temperature, has led to improvements in MDD symptoms in multiple randomized-controlled trials (RCTs). For instance, Janssen and Colleagues RCT displayed statistically significant improvements in Hamilton Depression Rating Scale (HDRS) scores compared to sham treatment following a single infrared sauna session conducted until the subjects’ core body temperatures reached 38.5 °C (101.3 °F). Two interesting findings particularly stick out in this study: 1. Subjects in the treatment group maintained statistically significant improvements in their (HDRS) scores for 6 weeks after just one infrared sauna session ; 2. 71.4% of subjects in the sham group reported that they believed they received the actual treatment.
“Supporting the credibility of our sham condition, 10 of 14 participants (71.4%) randomized to sham believed they had received active hyperthermia immediately on completion of the procedure (compared with 15 of 16 [93.8%] receiving active [whole body hyperthermia])” (X)
Though the studies mentioned above involve modalities other than traditional dry sauna, the presence of increased body temperature in each – paired with other data linking typical sauna methods with positive neural and mental health metrics, such as decreased risk of neurodegenerative disease – suggest that the non-traditional heat therapy data likely carries over to traditional dry sauna use. (I)
Health and Longevity
Thus far, we’ve addressed how heat therapy can impact healthspan through improvements in physical and cognitive metrics, and we’ll wrap up by addressing the relationship between sauna-bathing and lifespan. When it comes to longevity research focusing on sauna use, data from the Finnish Kuopio Ischemic Heart Disease Risk Factor Study (KIHD) – a large, prospective cohort study following middle-aged, Finnish men for 20+ years – and related publications take the cake.(I, VI, VII, VIII) Observing 2327 males aged 42-60, Laukkanen et al. found that, across ~18 years, using the sauna 2-3 and 4-7 times per week were respectively associated with statistically significant 22% and 63% relative risk reductions for sudden cardiac death (SCD) as compared to using the sauna 1 time per week; additionally, these same groups displayed statistically significant corresponding risk reductions of 23% and 48% for fatal coronary heart disease (CHD), 27% and 50% for fatal cardiovascular disease (CVD), and 24% and 40% for all-cause mortality (ACM) as compared to the group of subjects using the sauna once per week. (VI) Notably, all of these risk reductions exist after accounting for age, BMI, systolic blood pressure, serum low-density lipoprotein cholesterol level, smoking, alcohol consumption, previous myocardial infarction, type 2 diabetes mellitus, cardiorespiratory fitness, resting heart rate, physical activity, and socioeconomic status.
“After adjustment for CVD risk factors, the hazard ratio of SCD was 0.78 (95% CI, 0.57-1.07) for 2 to 3 sauna bathing sessions per week and 0.37 (95% CI, 0.18-0.75) for 4 to 7 sauna bathing sessions per week compared with men reporting 1 sauna bathing session per week (P for trend = .005)...Participants with a higher frequency of sauna bathing per week also had a lower risk of CHD and CVD mortality…the risk of fatal CHD events was 23% lower for 2 to 3 sauna bathing sessions per week and 48% lower for 4 to 7 sauna bathing sessions per week (P for trend = .005), respectively. Correspondent estimates for CVD mortality were 27% and 50% (P for trend < .001).” (VI)
Interestingly, duration of sauna sessions also demonstrated an impact, revealing statistically significant relative risk reductions of 52% for SCD, 36% for CHD, and 24% for CVD for subjects sauna bathing for greater than 19 minutes per session as compared to those bathing less than 11 minutes per session. (VI)
“Similarly, an inverse association was found between sauna bathing session duration and risk of SCD, with the risk 52% lower comparing participants in the top (>19 minutes per session) vs bottom (<11 minutes) groups of sauna bathing session duration.” (VI)
In addition to the main study, there are multiple spin-offs of the KIHD that found benefits from sauna bathing using the same subject data to assess other health metrics. For example, in Zaccardi and colleagues’ study, subjects using the sauna 2-3 and 4-7 times per week respectively displayed statistically significant 17% and 47% relative risk reductions for high blood pressure as compared to the group using the sauna once per week – even after adjusting for baseline age, smoking, body mass index, systolic blood pressure, blood glucose, blood creatinine, alcohol consumption, heart rate, family history of hypertension, socioeconomic status, and cardiorespiratory fitness. (VII) Also, Kunutsor et al.’s publication on the joint effects of cardiorespiratory fitness (CRF) and frequency of sauna bathing (FSB) in the KIHD population displayed statistically significant relative risk reductions in CVD of 58%, 50%, and 28% for subjects with high CRF and high FSB, high CRF and low FSB, and low CRF and high FSB, respectively, as compared to subjects with low CRF and low FSB. (VIII) The same groups respectively demonstrated statistically significant risk reductions in ACM of 40%, 37%, and 22% as compared to subjects with low CRF and low FSB. (VIII) Three takeaways in particular pop out to me here: 1. Sauna use reduces CVD and ACM even with low cardiorespiratory fitness ; 2. High CRF and high FSB offers greater benefit than high CRF alone; 3. Developing high CRF (measured by a symptom-limited VO2 max test in this study) alone is far more powerful than sauna use alone for CVD and ACM outcomes.
“During a median follow-up of 26.1 years, 520 cardiovascular and 1124 all-cause deaths occurred. Comparing high versus low CRF, the multivariate-adjusted hazard ratios (HRs) 95% CIs for cardiovascular and all-cause mortality were 0.51 (0.41–0.63) and 0.65 (0.57–0.75), respectively. Comparing high versus low FSB, the corresponding HRs were 0.74 (0.59–0.94) and 0.84 (0.72–0.97), respectively. Compared to low CRF & low FSB, the HRs of CVD mortality for high CRF & high FSB; high CRF & low FSB; and low CRF & high FSB were 0.42 (0.28–0.62), 0.50 (0.39–0.63) and 0.72 (0.54–0.97), respectively. For all-cause mortality, the corresponding HRs were 0.60 (0.48–0.76), 0.63 (0.54–0.74) and 0.78 (0.64–0.96), respectively.” (VIII)
All of these studies are limited, especially in that the KIHD subjects only reported their sauna use and lifestyle habits at baseline – they did not return later to confirm whether or not their habits changed over the ~2-decade durations of the studies. Not to mention, the population is entirely made up of middle-aged Finnish males. With that said, the large risk reductions, showing consistent dose-dependent benefits – i.e. the more sauna use the greater the benefits – are noteworthy, and even more so considering the swath of experimental and mechanistic data suggesting sauna use is an impactful tool for strengthening both healthspan and lifespan. (I)