Sauna and the heart: what the Finnish cohorts showed and where the link ends and the conjecture begins

The sauna often appears in biohacking not because there are robust trials on it, but because it has a compelling curve from Finnish cohorts: the more often people visited the sauna, the lower their cardiovascular mortality. The problem is that a beautiful curve does not yet prove causation. A cohort shows an association, but it does not answer what exactly created it.

Main data came from Finland

The most cited work is an analysis of the KIHD cohort, the Finnish Kuopio Ischemic Heart Disease Risk Factor Study. It included 2315 men aged 42–60 from Eastern Finland, who were asked once in 1984–1989 how often and for how long they used a sauna. They were then observed for a median of 20.7 years. Source: PubMed.

The gradient was striking. Among those who reported sauna use once a week, fatal cardiovascular events occurred in 22.3%, and all-cause mortality was 49.1%. In the group using the sauna 2–3 times a week, the figures were 16.4% and 37.8%. In the group using it 4–7 times a week, they were 12.0% and 30.8%. For sudden cardiac death, after adjusting for risk factors, the hazard ratio was 0.78 for the 2–3 times a week group and 0.37 for the 4–7 times a week group compared to once a week.

Later, the same group of researchers published another prospective study, this time including both men and women. It also found a link between more frequent sauna use and lower cardiovascular mortality. Source: PMC.

Why this is not proof of causation

A cohort study is simple: people are asked about a habit, and then for years, researchers observe who gets sick and who dies. This design is good at finding associations but cannot definitively separate cause from confounding factors. In the case of sauna, there are particularly many confounding factors.

• Baseline health. People with chronic symptoms, poor heat tolerance, weakness, or unstable angina are less likely to use a sauna or stop using it altogether.
• Reverse causality. This is particularly plausible here: it's not the sauna that makes a person healthy, but rather a healthier person is more likely to tolerate sauna as a regular part of life.
• Lifestyle. Frequent sauna use may go hand-in-hand with physical activity, sleep, social habits, income level, and a general lifestyle that is itself associated with the risk of death.
• Exposure was measured once. Researchers do not know exactly how the habit changed over the next twenty years.
• No true zero control. In KIHD, almost everyone used a sauna, and the reference group was not “never” but “once a week.”

Therefore, the correct interpretation of this cohort study is: frequent sauna use was a marker of lower cardiovascular mortality. The phrase “sauna reduces mortality” is stronger than the data allows.

What happens to the body in a sauna right now

Heat is a thermal stress. The skin dilates blood vessels, the heart has to pump blood faster, sweat carries away water, and blood pressure can drop after exiting. These effects are not hypothetical. In a meta-analysis of acute and short-term studies, after sauna use, the average heart rate increased by approximately 17.9 beats per minute, and systolic and diastolic blood pressure immediately after the procedure were lower by approximately 5.6 and 6.5 mmHg, respectively. Source: PubMed.

Such physiology explains why sauna is sometimes compared to moderate exercise without muscle work. However, an acute drop in blood pressure does not imply that years of use inherently prolong life. The same gap exists between short-term effects and long-term mortality as in other biohacking narratives involving surrogate markers.

Where the risk begins

The flip side of the same physiology includes fainting, hypotension, dehydration, heat exhaustion, and, in vulnerable groups, arrhythmias. In a recent review on acute heat exposure, sauna is discussed alongside hot baths and hot springs as a single class of stressors: vasodilation, relative hypovolemia, cardiovascular stress, electrolyte disturbances, falls, drownings, and aspiration after loss of consciousness. Source: PubMed.

This is directly related to the topic of water and salts: fluid loss in heat does not make sauna a “detox” but simply shifts plasma volume and circulatory load. A discussion of physiology is more appropriate here than slogans — see the article on water and electrolytes.

Alcohol in the sauna is a separate risk factor

In the literature, alcohol does not appear to be a minor everyday detail. In a review on sudden cardiac death, the authors specifically write that reports of deaths in saunas were linked to dehydration, hypotension, and arrhythmias against the background of combined heat and alcohol. Source: PubMed.

An older review specifically dedicated to the combination of sauna and alcohol described annual deaths in Finland where alcohol was considered a significant contributing factor: fainting, heatstroke, injuries, and drownings after sauna use. The authors honestly noted that direct physiological studies were few, but the signal regarding accidents was consistent. Source: PubMed.

Who tolerates heat less well

In reviews on safety and thermotherapy, high-risk groups include people with unstable cardiovascular conditions, severe aortic stenosis, recent myocardial infarction or stroke, marked predisposition to orthostatic hypotension, seizure disorders, and conditions where fluid loss rapidly worsens hemodynamics. For stable, medically controlled conditions, the picture is milder: an older review on sauna risks describes good tolerance in most healthy individuals and in some patients with stable cardiovascular disease. Source: PubMed.

That is, “sauna is good for the heart” and “sauna is safe for everyone” are two different phrases, and the second is clearly incorrect.

Why sauna fit so well into biohacking

Sauna has a convenient narrative. There's an ancient practice, a pleasant subjective feeling after exiting, beautiful observational graphs, and clear physiological language about blood vessels, cardiac output, and heat shock proteins. All of this sells very well as a “protocol.” But the hierarchy of evidence does not change because of this: the strongest evidence here is not clinical trials, but observations.

Therefore, sauna is relevant in the biohacking section not as a proven way to prolong life, but as a good example of how easily association can be confused with causation. This is the same analysis of perspective as in the article on biohacking in general. And next to cold, sauna looks particularly illustrative: both have striking physiology, but a long path from acute effects to a discussion of health — see the article on cold and brown fat.

Count it from a photo

Frequently asked questions

Is it true that saunas reduce cardiovascular mortality?

Finnish cohort studies have shown a consistent association: the more frequently participants reported using a sauna, the lower their cardiovascular mortality. However, these are observational data, not a randomized trial, so they do not prove causation.

What is reverse causality in this context?

This is a scenario in which healthier people are more likely to use a sauna, rather than the sauna making them healthier. This bias is particularly likely in this topic because poor heat tolerance itself reduces the frequency of visits.

Does the sauna have acute physiological effects?

Yes. After heat exposure, heart rate and skin blood flow typically increase, and blood pressure may drop after exiting. These shifts have been measured in clinical studies and meta-analyses.

Why is alcohol discussed separately in relation to saunas?

Because in the literature, this specific combination is associated with fainting, injuries, drownings, and some sudden deaths. Heat, vasodilation, and alcohol together destabilize hemodynamics and behavioral control more significantly.

Who tolerates saunas less well?

Reviews list people with unstable cardiovascular conditions, severe aortic stenosis, recent heart attack or stroke, significant orthostatic hypotension, seizure disorders, and conditions that deteriorate rapidly with fluid loss as high-risk groups.

Read next

• Biohacking: separating what has been tested from what is being sold — There is no definition of biohacking that two people would both sign.
• Cold exposure and brown fat: how many calories an ice bath actually costs — Brown fat is one of the rare cases where the elegant mechanism turns out to be real: the tissue genuinely converts fat into heat, bypassing the usual route.
• Water and electrolytes during training: when plain water is not enough — Plain water is enough for almost everyone, almost all the time.

This article is for general information. It is not medical advice, a diagnosis, or a prescription for treatment or a diet, and it does not replace a consultation with your doctor. If you have a health condition, are pregnant, take medication, or follow a diet prescribed to you, decisions about food belong with your doctor.

Figures from regulations, guidelines and studies are given as they stood when this article was prepared and may since have changed; check them against the primary sources. This article is not advertising, an offer, or individual advice, and neither the author nor the site owner is responsible for decisions taken on the basis of it.