Is the sauna good for runners? What the evidence supports, and what it doesn’t

Sauna research has produced one genuinely arresting set of numbers and one badly mangled headline. Both are worth understanding before you decide whether to bother.

The Finnish mortality findings

Researchers followed 2,315 middle-aged men in Eastern Finland for a median of 20.7 years, recording how often they used a sauna. Compared with men who went once a week, those going four to seven times a week had a hazard ratio for sudden cardiac death of 0.37 (95% CI 0.18 to 0.75). Session length mattered too, for that same outcome: sessions over 19 minutes carried a hazard ratio of 0.48 (0.31 to 0.75) against sessions under 11 minutes — note that is long sessions versus short ones, not against no sauna at all.

A later analysis of 1,688 people — this time 51.4% women, followed for a median of 15 years — found cardiovascular mortality hazard ratios of 0.23 (95% CI 0.08 to 0.65) for the most frequent users after full adjustment.

Why you should hold those numbers loosely

They are striking. They are also weaker evidence than they appear, for reasons worth spelling out:

• They are observational. Nobody was assigned to use a sauna. This shows an association, not that sauna use caused the difference. The authors’ own conclusion says exactly that, and calls for further studies “to establish the potential mechanism”.
• It is essentially one cohort. The 2015 and 2018 papers draw on the same Finnish study population and largely the same research group. They are not two independent confirmations, and presenting them as such would overstate the case.
• Reverse causation is a live problem. People who are already unwell use the sauna less. The authors name this as an unresolved limitation.
• Some intervals are very wide. A hazard ratio of 0.23 with a confidence interval running from 0.08 to 0.65 spans an eightfold range — that is a signal, not a precise estimate. The most-frequent-user group contained few people and few events.
• Sauna use in Finland travels with other things — affluence, leisure time, social connection — that are hard to fully adjust for.

None of this makes the finding worthless. It makes “saunas reduce your risk of dying” the wrong sentence, and “frequent sauna use was associated with lower mortality in this Finnish cohort” the right one.

And then a trial tested it directly

All of the above is observational. In 2023 researchers finally ran the randomised test: 41 adults with stable coronary artery disease were assigned either to eight weeks of sauna — four sessions a week, 20 to 30 minutes at 79°C — or to carrying on as normal.

The sauna group did physiologically acclimatise: lower resting core temperature, higher sweat rate. The dose worked. But on the vascular measures the observational studies point at, nothing changed. Flow-mediated dilation shifted 0.15% in the sauna group against 0.07% in controls (interaction p = 0.909). Arterial stiffness: no difference (p = 0.816). Systolic blood pressure: no difference (p = 0.951). The authors conclude that this dose of sauna bathing “does not improve markers of vascular health in adults with stable CAD”.

This is the most important study on this page, and it cuts against the headline. A strong association across twenty years of Finnish data, and a null result when the mechanism was actually tested. That pattern — impressive observational signal, disappointing trial — is extremely common in health research, and it is usually the trial that turns out to be right.

The 32% claim, corrected

You will see it stated that sauna use improves endurance by 32%. That number comes from a study of six male distance runners who sat in a sauna at about 90°C for roughly 31 minutes after training, around 13 times over three weeks.

Run time to exhaustion did increase by 32%. But time-to-exhaustion tests exaggerate performance changes dramatically, and the authors themselves translate it: equivalent to roughly 1.9% (90% confidence limits 1.3–2.4%) in an actual endurance time trial. A 1.9% improvement is genuinely meaningful in racing. It is not 32%, and quoting the larger number misrepresents the paper.

What did move clearly was plasma volume, up 7.1%. That is the plausible mechanism: heat exposure expands blood plasma, which improves cardiovascular efficiency — the same adaptation that makes heat acclimatisation work.

A stronger study on that mechanism found 10 days of heat acclimation raised VO₂ max by 5% in cool conditions and improved time-trial performance by 6%. Two things to note before you cite it: those were cyclists, not runners, and only 12 of them — and the protocol was exercising in a 40°C chamber, not sitting in a sauna. It supports the mechanism, not the sauna specifically.

Then in 2025 someone pooled all of it. A meta-analysis of ten controlled studies covering 199 participants looked at exactly this intervention — heat exposure after training — and found the effect on endurance performance was trivial and not statistically significant (ratio of means 1.04, 95% CI 0.94 to 1.15, p = 0.46). The authors graded the certainty of the evidence as low to very low, citing small samples, risk of bias, and risk of publication bias, and concluded that using post-exercise heat to improve performance is “uncertain”.

So the fair reading is not “about 2%”. It is that a single six-person study suggested a small benefit, and when everything available was pooled, the benefit did not hold up.

What is fair to say

• Heat exposure expands plasma volume. Well measured, consistent across studies, and physiologically sensible — this is the best-supported claim on the page.
• The cardiovascular benefit is unproven. Strongly suggested by Finnish cohort data, and not found when a trial went looking for it.
• Sauna is well tolerated by most healthy adults, including many with stable coronary heart disease.
• It may help you handle Indian summer running — heat acclimatisation is real, and if you race in May in NCR you are running in heat whether you prepared for it or not.
• It probably will not transform your race times. The best estimate from the only study in runners is around 2%.

The practical answer

If you enjoy it and have easy access, the case is reasonable: plasma volume expansion is real, heat tolerance is genuinely useful in an Indian summer, and the safety profile for healthy adults is good. If you are considering building your training around it, or paying substantially for access, the evidence does not support that — and unlike cold plunges, at least nothing here suggests it works against your training.

References: Debray A, et al. Finnish sauna bathing and vascular health of adults with coronary artery disease: a randomized controlled trial. J Appl Physiol, 2023. doi:10.1152/japplphysiol.00322.2023 · Laukkanen T, et al. Association between sauna bathing and fatal cardiovascular and all-cause mortality events. JAMA Intern Med, 2015. doi:10.1001/jamainternmed.2014.8187 · Laukkanen T, et al. Sauna bathing is associated with reduced cardiovascular mortality. BMC Med, 2018. doi:10.1186/s12916-018-1198-0 · Scoon GS, et al. Effect of post-exercise sauna bathing on the endurance performance of competitive male runners. J Sci Med Sport, 2007. doi:10.1016/j.jsams.2006.06.009 · Lorenzo S, et al. Heat acclimation improves exercise performance. J Appl Physiol, 2010. doi:10.1152/japplphysiol.00495.2010 · Hannuksela ML, Ellahham S. Benefits and risks of sauna bathing. Am J Med, 2001. doi:10.1016/s0002-9343(00)00671-9