Clinical Effects of Regular Dry Sauna Bathing: A Systematic Review

Executive summary

If you want one entry that tells you how strong the sauna evidence base actually is, this is it.

The authors searched medical databases for studies reporting health effects of regular dry sauna bathing in humans from 2000 onwards, and assessed risk of bias using Cochrane Collaboration guidelines. Forty clinical studies involving 3,855 participants met the inclusion criteria.

Most of those studies reported beneficial health effects. But only 13 were randomised controlled trials, and most studies were small, with fewer than 40 participants. Reported outcome measures were heterogeneous, which prevented pooling.

Only one small study (n=10) reported an adverse health outcome: disrupted male spermatogenesis, demonstrated to be reversible when sauna activity ceased. The authors note that more data of higher quality is needed on the frequency and extent of adverse side effects - in other words, the safety literature is thinner than the benefit literature.

Their conclusion is measured: regular dry sauna bathing has potential health benefits, and further study is needed to determine optimal frequency and duration for targeted effects and which clinical populations are most likely to benefit.

Why this research matters

The paper opens by observing that many health benefits are claimed by individuals and facilities promoting sauna bathing, while the medical evidence to support these claims is not well established. That sentence is the reason this entry is in the library.

Individual studies tell you what one group found. A systematic review with formal bias assessment tells you how much confidence the field as a whole can carry. For a buyer weighing marketing claims, the second is far more useful.

What the researchers found

Forty studies with 3,855 participants met inclusion. Most reported beneficial health effects across a heterogeneous set of outcome measures.

Only 13 of the 40 were randomised controlled trials. Most studies were small, with fewer than 40 participants each.

One small study of 10 participants reported an adverse outcome - disrupted male spermatogenesis - which was reversible when sauna use stopped. The review notes that the majority of studies did not systematically report adverse events.

Because outcomes were heterogeneous, no pooled effect sizes, confidence intervals or p-values were generated. Individual study statistics are not reproduced in the review abstract and are therefore not restated here.

What the results mean

'Most studies reported beneficial effects' sounds stronger than it is. Small studies with heterogeneous outcomes and no pooling are exactly the conditions in which positive findings accumulate whether or not an effect exists, because small negative studies are less likely to be published at all.

What is reasonably supported is that regular dry sauna bathing has potential health benefits and appears well tolerated by most people who use it. What is not supported is any specific claim about a specific outcome at a specific dose - the review states plainly that optimal frequency and duration have not been determined.

The adverse-event finding deserves attention precisely because it is nearly the only one. A literature that reports benefits enthusiastically and harms rarely is not a literature that has looked hard at harms.

Strengths and limitations

Strengths

• Systematic methodology with a defined search strategy and inclusion criteria.
• Formal risk-of-bias assessment using Cochrane Collaboration guidelines.
• Reasonable total sample across the field, at 3,855 participants.
• Explicitly reports the proportion of randomised trials rather than treating all studies as equivalent.
• Reports an adverse outcome and flags under-reporting of harms rather than omitting the question.
• Candid conclusions that do not overstate the evidence.

Limitations

• No meta-analysis, so no pooled effect estimates are available.
• Only 13 of 40 included studies were randomised.
• Most included studies were small, with fewer than 40 participants.
• Heterogeneous outcome measures across studies.
• Restricted to studies published from 2000 onwards.
• Adverse events were not actively surveilled in most included studies.
• Sauna type, temperature and protocol varied across included studies and were not standardised.

Evidence in context

This review sits alongside the Mayo Clinic Proceedings reviews and the Kuopio cohort analyses as the main pillars of the sauna evidence base. It is the most useful of them for judging quality rather than direction, because it is the one that counted the randomised trials and assessed bias formally.

Its conclusions have not been overturned. Later reviews, including a 2023 review examining sauna bathing in combination with other lifestyle factors, have added breadth without resolving the central limitations this review identified: small studies, heterogeneous protocols, and undefined optimal dosing.

The field remains one where the direction of evidence is fairly consistent and the strength of evidence is modest.

Related research

1. Cardiovascular and other health benefits of sauna bathing: a review of the evidence Laukkanen JA, Laukkanen T, Kunutsor SK. Mayo Clinic Proceedings 93(8):1111-1121. 2018. PMID 30077204; doi:10.1016/j.mayocp.2018.04.008.A broad review of the same field, more favourable in tone and less focused on study quality.
2. Association between sauna bathing and fatal cardiovascular and all-cause mortality events Laukkanen T, Khan H, Zaccardi F, Laukkanen JA. JAMA Internal Medicine 175(4):542-8. 2015. PMID 25705824; doi:10.1001/jamainternmed.2014.8187.The largest and most cited individual study in this literature, and an example of the observational designs that dominate it.
3. Far-infrared saunas for treatment of cardiovascular risk factors: summary of published evidence Beever R. Canadian Family Physician 55(7):691-6. 2009. PMID 19602651; PMC2718593.The equivalent exercise for far-infrared cabins specifically, and a much smaller literature.
4. Does the combination of Finnish sauna bathing and other lifestyle factors confer additional health benefits? A review of the evidence Kunutsor SK, Laukkanen JA. Mayo Clinic Proceedings 98(6):915-926. 2023. PMID 37270272; doi:10.1016/j.mayocp.2023.01.008.A more recent review examining combined lifestyle exposures.
5. Before-bedtime passive body heating by warm shower or bath to improve sleep: a systematic review and meta-analysis Haghayegh S, Khoshnevis S, Smolensky MH, Diller KR, Castriotta RJ. Sleep Medicine Reviews 46:124-135. 2019. PMID 31102877; doi:10.1016/j.smrv.2019.04.008.A meta-analysis of passive body heating and sleep, methodologically stronger on a narrower question.