Two ideas get blended together a lot in longevity circles. One is that regular sauna use is linked to living longer. The other is that VO2 max, your aerobic ceiling, is one of the best predictors of how long you'll live, and that easy “zone 2” cardio is how you build the base under it. Put them together and you get a tempting shortcut: if a sauna pushes your heart rate into zone 2, maybe sitting in one counts as the workout.
That is the hypothesis I wanted to question. Here is what each piece of the research actually shows.
The Finnish sauna study
The study everyone cites comes from the Kuopio Ischemic Heart Disease Risk Factor Study in eastern Finland. Researchers followed 2,315 men aged 42 to 60 for a median of 20.7 years and compared how often they used a sauna with how they died. It was published in JAMA Internal Medicine in 2015.
Compared with men who used a sauna once a week, men who went 4 to 7 times a week had:
- 40% lower risk of death from any cause (hazard ratio 0.60)
- 50% lower risk of fatal cardiovascular disease (0.50)
- 48% lower risk of fatal coronary heart disease (0.52)
- 63% lower risk of sudden cardiac death (0.37)
Two or three sessions a week sat in between, at about 24% lower risk of death from any cause. Session length mattered for the heart outcomes too: sessions longer than 19 minutes went with fewer cardiac deaths than sessions under 11 minutes, though not with fewer deaths overall.
Keep the limits in view. This is an observational study, so it shows an association, not cause and effect. Men healthy enough to sauna every day may differ in ways the statistics can't fully remove. It included only middle-aged Finnish men, and it describes traditional Finnish dry saunas, about 79°C (174°F) on average for the most frequent users. It doesn't tell us about infrared cabins, steam rooms or hot tubs.
Why VO2 max is the number to watch
VO2 max is the most oxygen your body can take in and use during all-out effort. It reflects how much blood your heart can pump and how well your muscles can use the oxygen it carries. Doctors often call the broader measure cardiorespiratory fitness.
The largest study of it I know of came from the Cleveland Clinic, published in JAMA Network Open in 2018. It followed 122,007 patients who took a treadmill test between 1991 and 2014, for a median of 8.4 years. After adjusting for age, sex, other illnesses and medications, the least-fit group had about 5 times the risk of death of the fittest group (adjusted hazard ratio 5.04). The authors found that low fitness carried a risk comparable to or greater than smoking, diabetes or coronary artery disease. Two caveats: fitness there was estimated from treadmill performance rather than measured directly, and again this is an association.
Fitness is one of the few big predictors you can change, which is exactly why it is tempting to look for easier ways to move it.
Does a sauna mimic zone 2?
On the surface, yes. In a 2019 study of 19 volunteers published in Complementary Therapies in Medicine, a 25-minute sauna raised heart rate and blood pressure about as much as a short cycling test at 100 watts, a moderate effort. Both fell below their starting point during recovery.
The trouble shows up when you ask where the blood goes and how the heart produces it. A 2001 review of sauna physiology in the American Journal of Medicine lays it out:
| During a sauna | What happens |
|---|---|
| Heart rate | Up to about 100 beats a minute in regular users at moderate heat, and up to 150 in intense sessions or people new to it |
| Heart output | Rises from about 5–6 to 9–10 liters a minute, almost entirely from the faster heart rate. The amount pumped per beat (stroke volume) stays essentially unchanged |
| Blood to the skin | Jumps from 5–10% of the heart's output to 50–70%, to shed heat |
| Blood to muscle | Falls slightly, by about 0.2 liters a minute, along with less flow to the kidneys and gut |
Now compare that with what zone 2 training is for. Zone 2 is steady, conversational effort that you can hold for a long time. Its value comes from two things happening at once: your working muscles keep pulling oxygen out of the blood and burning fuel aerobically, and over weeks your heart adapts to pump more blood with each beat. Those are the adaptations that raise VO2 max.
A sauna does neither. The extra blood goes to your skin, not your legs, and the heart gets there by beating faster, not by filling and emptying more. Your muscles are resting.
Where heat does seem to help
None of this means heat is useless for fitness. The better evidence points to heat as a separate stimulus that adds to training.
A randomized trial from the same Finnish group. In 2022, researchers randomly assigned 47 inactive adults (average age 49), each with at least one cardiovascular risk factor, to one of three groups for 8 weeks: guideline-based exercise followed by a 15-minute sauna, the same exercise with no sauna, or neither. Both exercise groups got fitter. Compared with exercise alone, the exercise-plus-sauna group gained an extra 2.7 ml/kg/min of VO2 max, had systolic blood pressure 8 mmHg lower, and had lower total cholesterol. It was published in the American Journal of Physiology. It is a small trial and the range of possible effects is wide (the VO2 max gain could plausibly be anywhere from 0.2 to 5.3), but it is a controlled comparison, which the mortality studies are not.
Easy training in the heat. A 2010 University of Oregon study in the Journal of Applied Physiology had 12 trained cyclists ride at an easy pace, about 50% of their VO2 max, in a 40°C (104°F) chamber for 10 days. Their VO2 max rose about 5% when tested in cool conditions and about 8% in the heat. Their plasma volume (the liquid part of the blood) grew about 6.5%. A control group of 8 cyclists doing identical work in a cool room showed no change. The authors chose that easy pace precisely because it is too light to improve fitness on its own, which points to the heat as the cause.
Fitness plus sauna, over 26 years. Going back to the Finnish men, a 2018 analysis in Annals of Medicine looked at 2,277 of them whose fitness had been measured directly, followed for a median of 26.1 years. It split them by fitness (above or below the median) and by sauna habit (3–7 sessions a week versus 2 or fewer):
| Group | Risk of death, any cause | Risk of cardiovascular death |
|---|---|---|
| Low fitness, infrequent sauna | Reference | Reference |
| Low fitness, frequent sauna | 22% lower (0.78) | 28% lower (0.72) |
| High fitness, infrequent sauna | 37% lower (0.63) | 50% lower (0.50) |
| High fitness, frequent sauna | 40% lower (0.60) | 58% lower (0.42) |
Read the table carefully. The combination came out best, as the authors note. But being fit did far more of the work than the sauna did: high fitness alone captured most of the benefit, and adding frequent sauna on top moved the numbers only a little further. That fits the “add-on, not substitute” reading.
The likely mechanism, across these studies, is that repeated heat exposure expands plasma volume and conditions the blood vessels, which helps the heart deliver more blood when you do exercise. That is a different route from what training does inside the muscle.
What about heat and the muscle itself?
There is one intriguing exception to “heat doesn't touch the muscle.” In a 2018 study from Brigham Young University in the Journal of Applied Physiology, researchers heated one thigh muscle directly in 20 adults (10 men, 10 women) using a deep-tissue heating device, raising the muscle's temperature by about 3.9°C, for 2 hours a day over 6 days. The heated muscle showed higher capacity in its mitochondria (the cell's energy producers) and more PGC-1α, a protein that drives the building of new mitochondria.
That is the kind of change endurance training produces, which is interesting. But it doesn't rescue the sauna-as-zone-2 idea: it took hours of targeted heating to warm the muscle itself, and a 20-minute sauna isn't known to heat your leg muscles that way. Treat it as a reason for more research, not a reason to skip the workout.
Don't do your cardio inside the sauna
The obvious next thought is to combine the two and ride or walk inside a sauna. As far as I can find, no study has tested exercise inside a traditional Finnish sauna, and there are good reasons it hasn't been. The research above used either a sauna after exercise or easy exercise in moderate, controlled heat (around 40°C, not 80–90°C). Adding exercise to a hot room stacks two heat loads, and overheating, dehydration and fainting are real risks. The versions with evidence behind them are the ones to copy, with medical advice first.
What I take from it
- A sauna is not zone 2. It matches zone 2's heart-rate number, not its training effect. If your cardio time is being replaced by sauna time, the research doesn't support the swap.
- Heat looks like a useful add-on. The strongest evidence, the randomized trial, is for a short sauna after regular exercise. That is the pattern I'd look at first.
- Fitness carries most of the weight. In the long-term Finnish data, being fit mattered more than sauna habit. Do the work, then add the heat.
- Don't steer by heart rate in the heat. Heat inflates heart rate, so judge easy training by pace, power or how you feel, not heart rate alone, when it's hot.
- Talk to your physician first, especially if you have a heart condition, low or high blood pressure, take blood pressure medication, or are pregnant. Regular sauna use is not risk-free for everyone.