Data: daily records from wearable users who logged sauna sessions via connected apps. Within-person design — each user is their own control, comparing their own sauna-day nights against their own non-sauna-day nights. No cross-user comparisons.

Stats: paired t-tests, FDR-corrected p < 0.05, Cohen's d > 0.2 threshold for "meaningful effect." Anything below d=0.2 we don't report as a finding.

What we measured: minimum nighttime HR, max and average HR, HRV, activity minutes and distance, menstrual cycle phase (for female subset).

What we found: - On sauna days, minimum nighttime HR drops ~3 bpm (~5%) vs. the same user's non-sauna days. - Effect survives controlling for activity level. It's not "sauna users just exercised more that day." - Strongest hypothesis: elevated parasympathetic tone from the post-sauna cooling phase carries into sleep. Consistent with heat-stress physiology literature. - Sex difference: for women, the nighttime HR effect only crosses the d > 0.2 threshold during the luteal phase. No meaningful effect during the follicular phase. We didn't expect this; worth replicating.

What we can't control for: - Sauna type (dry / infrared / steam), duration, temperature. Not captured. - Dose-response. We don't know session length per user. - Timing of sauna relative to sleep. - Reverse causation: people may sauna on days they already feel recovered. - Selection: wearable users who bother logging sauna are a health-conscious cohort.

What surprised us: the effect is larger than what we see for comparable-intensity exercise days. If you treat nighttime HR as a parasympathetic recovery signal, sauna beats a moderate workout on the same user. Not what I'd have predicted.

My current guess is no. That is this improves a marker for good health without improving health. However this is a guess by someone who isn't in the medical field and so could be wrong.

My take is that your heart and lungs are working out, even if your body is not. Do you get the same benefits as going for a run or bike ride for a comparable amount of time? no, since your limbs don't get fit, but your heart and lungs do.

Endurance athletes obsessively track VO2 max, basically your body's ability to consume oxygen during workouts, and it certainly doesn't improve with sauna training.

It's like asking "if you only did puzzles, would you be smarter?" Well, in a way, yes, but if you actually want to compete with someone with a good education you have to read.

Same with physical exercise. It puts a lot of different stresses on your body that saunas don't. The question isn't "do saunas make you physically fit," because they don't. The question is "for people who don't want to exercise, does sauna training alone meaningfully extend your healthspan?" I'm guessing the answer is "a little but not enough," but I'm not sure.

Yucatan is not the same as Dubai in Summer.

Your body is under heat shock trying to keep up in a Sauna (that isn't considered warm until 60°). Versus a healthy body CAN keep up in 40°.

The Yucatan equivalent of a Sauna is more like doing hard labor on a roof on a sunny day with no breeze.

Tamales, Cochinita (roasted pork with herbs), salbutes, trancas. Everything of course cooked in Lard. With CocaCola on the side.

So yeah, that's a strong point.

Please ignore my comment, though I will leave it to make the below comments less confusing.

Original: You don't want to "work out" your heart though. Cardiac hypertrophy is a bad thing.

The benefit of exercise is that your muscles become more oxygen-efficient. Your heart endures some stress now, so that it can work less in the future.

Eccentric hypertrophy (athlete's heart) is the positive adaptation resulting from training the heart. The heart has a lower resting rate and is more efficient at pumping blood. It returns to normal size if training stops.

You'll never reach a state of hypertrophic cardiomyopathy (the bad kind of hypertrophy) with exercise. Its cause is usually genetic.

That additional oxygen needs to come from somewhere. Endurance training at the same time trains the heart to deliver more oxygen to the periphery; the primary mechanism is increased cardiac stroke volume.

Cardiac hypertrophy isn't a "bad thing". This is completely contextual. What you don't want, for example, is pathological hypertrophy from things like hypertension, or exclusive left ventricular hypertrophy without associated increase in chamber size.

The heart is very complex. You 100% should exercise it.

Afaict, the grand parent poster is just very wrong. You do want to cause acute stresses to your heart (cardiovascular exercise) to get it work better.

Sources?

If heat training is better than another interval session remains to be seen but it seems a lot of smart people believe it's worth it nowadays.

Agreed on the long-term effect too: doing a study on long term health is a completely different story

There is a substantial body of existing research to peruse about the impact of regular sauna use on health outcomes, much of it from Finland given the prevalence of sauna usage there allowing for larger sample sizes. It's a body of evidence rather than one knock-out experimental design.

Smoke saunas a bit less, electric or wood stove saunas no issue. It's nice to take a breather once in a while but I'd honestly have no issues sitting in a 80-90 deg sauna for an hour as long as I have enough to drink with me.

One time I sat in the sauna for six hours with a few breaks between with a group of friends shooting the shit. I had a headache the next morning but I blame it on the Jallu and not the sauna.

If 10mins feels too much, do less.

For example: body temp increases, heart rate increases, and we sweat. But the muscles aren't "engaged", consuming stuff (glycogen, etc.) while doing sauna.

There could also be sauna benefits that exercise does not impart, or is less likely to do so: sweating greater than exercise could lead to excess excretion of plastics, carcinogens, etc.

Running in mild/cold temps we do little sweating (unless long duration exercise), whereas every darn sauna at sufficiently high temps we are going to be sweating.

Nordic people tend to live a long life even though they historically didn't have access to fresh vegetables or fruit and brutal winters (and darkness) prohibited excercise.

ps. I'm not arguing that excercise is unhealthy, it's just that its contribution to eventual longevity, is currently unknown. Whereas anectodal evidence of saunas (being around longer than "excercise"), seems to work.

I see numerous studies indicating that exercise contributes directly to eventual longevity, e.g.:

https://www.ama-assn.org/public-health/prevention-wellness/m...

https://www.acc.org/latest-in-cardiology/articles/2025/07/02...

https://pmc.ncbi.nlm.nih.gov/articles/PMC3395188/

I do wonder what the correlation is: is it only because of excercise, or at least partially also due to the fact those who can set aside time and effort (and often, money) to exercise, have a "better" life than those who don't?

For example, high life expectancy in Madrid, and Switzerland are often attributed to having broad access to great healthcare and stress-free lifestyle(both), despite living a relatively "unhealthy" lifestyle, at least in Madrid. Eating fried food everyday, little exercize among elderly (at least if you don't count walking to the bar). Those 85 year+ Madrileños probably had their last formal exercise when they had to do their military service back in the day.

As in the case of top athletes, in your second article, is their longevity due to heavy exercise, or kind of, "despite it", and at least partially due to their accumulated wealth, health-conscious mindset plus the ability to afford a stress-free life?

So it could be that exercise helps keep this elasticity, the same way maybe sauna does? Also antioxidants from vegetables etc.

So it could be that it is a _factor_, but definitely needs way more study.

I am also not in the medical field, but I think arteriosclerosis is a well known link for cardiovascular disease.

Put differently, relying on self reporting for any sort of status from people is just not a reliable methodology.

Not exercising as in sports and not exercising, period, are very different. If you look at the American blue zone, those people are certainly exercising; daily nature walks are baked into their theology.

https://www.science.org/content/article/do-blue-zones-suppos...

When I'm in my dry sauna and really pushing myself with the heat and steam off the hot rocks, I basically have to mediate to stay in beyond 15 minutes because every part of my mind starts telling me to get out and cool down.

How did you control for activity level? Do you have similar BPM plots for the different situations (sauna+exercise, sauna+no exercise, no sauna + exercise, no sauna + no exercise) for a visual representation?

> minimum nighttime HR drops ~3 bpm (~5%)

What wearables were used? These devices don't usually have enough precision to reliably detect ~3bpm changes. Also, the measurements are sensitive to skin, blood flow changes and temperature. How do you know the difference doesn't come from different sensor behavior after sauna?

For large sample averages this doesn't really matter.

Precision (inverse of variance) of estimate of mean increases directly proportional to number of samples (given some assumptions that very likely hold here). If you have measurement standard deviation of say 10 bpm, with 100 measurements you have mean estimate standard deviation of 10/sqrt(100) = 1 bpm.

But you can't really assume that the estimate of the mean represents the real value. For example, if the sensor is equally likely to show 80 or 81 BPM when the real heartrate is 80.7, the mean estimator will be biased.

> with 100 measurements

Also, wearables aren't taking 100 measurements of the BPM at a given point in time. I think the highest frequency they usually have is 1 second measurement interval. So they don't really have a lot of measurements for each point in time.

> mean estimate standard deviation

That's the standard deviation of the mean of the values. Doesn't imply that the standard deviation of the values themselves will go to zero.

> I don't understand what you mean by that.

That as a rule of thumb, you should not assume that repeating measurements will give you more precision than what the tool can offer. E.g., trying to measure down to milimeters with a ruler that has only 1cm marks will not really work well.

Bias is different from precision. If both conditions have the same bias, their difference is still unbiased.

> Also, wearables aren't taking 100 measurements of the BPM at a given point in time. I think the highest frequency they usually have is 1 second measurement interval. So they don't really have a lot of measurements for each point in time.

I did not mean taking multiple measurements in succession. Those are likely to have correlated noise, meaning the assumptions do not hold. But between participants measurement noise is very unlikely to be correlated.

> That as a rule of thumb, you should not assume that repeating measurements will give you more precision than what the tool can offer. E.g., trying to measure down to milimeters with a ruler that has only 1cm marks will not really work well.

If you quantize so much that you have no variance in the measurements, then sure. But watches typically have 1 bpm quantization, which is fine at the scale of variation in HR.

If you have independent error in measurements and quantization that gives you variance in measurement, you very much can assume repeating measurements will give you more precision than the tool can offer. This is how e.g. particle physics (and many many other fields of science) is done.

- Is the wearable accurate enough to be sure that 3bpm is not a measurement fluke? - Why did you use the minimum heart rate value (which could be a measurement glitch) and did not compare a percentile (e.g., 2.5th lowest percentile)? - Were all assumptions for paired t-testing valid? How did you account for likely temporal correlations in the data (e.g., sauna could have an effect also on a night 2 days after it, same for exercise)? - How can you define a "comparable-intensity exercise day" if you don't know the characteristics of the sauna?

If the statistical tests show significance (and are valid), the answer to this question is yes. If you have enough data you can make strong conclusions even witwith imperfect hardware.

- How was the controlling for the other factors done? A linear model?

- What were the sauna vs non-sauna baseline HRs in fig 1? Could you plot raw averages?

- Was the min HR explicitly computed during the night (in Fig 2), or was it assumed min HR occurs during the night?

- Reporting only significant results is not prudent even with multiple comparisons corrections, please report all tests made

Exercise, over time, should lower the baseline (to a point). I’d think this would have the more desirable long term benefits.

One can do both, of course, but when people see headlines like this they often jump to the conclusion that sauna can replace exercise, because that’s what they want to believe.

> What we can't control for: - Sauna type (dry / infrared / steam), duration, temperature. Not captured

Could probably capture humidity/duration/temperature using a sensor in wearable device...

It seems you ask participants to log if they went to sauna. Out of curiosity, why is it not simple to also ask for a type?

The sauna might be acting like any other drug. There are a lot of drugs that will lower nighttime heart rate. Does that mean those drugs are healthier for you?

So a hot shower before bed is actually great for sleep, because you get the increased skin temp, relaxed muscles from the warm water, and general relaxation because showers are (for many people) relaxing.

The headline claim is very misleading for anyone who thought there were 59,000 people in this data set.

The absolute difference is also small. Small enough that the effect might be attributable to something secondary, such as sauna users consuming more water in recovery and being more hydrated. Heart rate has a relationship with hydration status.

Also, my samsung watch can measure stress (whatever it means). It always shows the very, very minimal stress for me. The only time that I have been stressed was the day that I spent a bit too much on the steam room.

Zone 2 is great but the best health outcomes are from people who do high intensity exercise interspersed with zone 2 exercise.

Some people find it gross to basically sweat inside a powered sleeping bag, but if you don't mind that you can get the same effects of a sauna while lying on your (covered) couch and watching YouTube.

https://cptsd.sites.umassd.edu/bryan-johnson-and-microplasti...

Is having a lower night time heart rate the core goal of exercise? Is it even a goal at all? Or is it just an indicator of other goals being reached? I'm genuinely curious, I wasn't aware that the number mattered, more than what that number actually represents.

From the author, "Strongest hypothesis: elevated parasympathetic tone from the post-sauna cooling phase carries into sleep"

AKA, they use it as a proxy to infer a deeper state of rest and improved recovery state. Says nothing about the fatigue generated from using a sauna.

Maybe the conclusion is correct, or maybe not, but as written the methodology is under specified, statistics are not supported, and there too many confounders not addressed. One should not take anything from this without a better write up. Just misunderstanding what n= means is a huge flag.

Since the author is here, I have to ask: Why a blog post and not an actual paper? Why spray this onto the internet without validating the work? Or, conversely, why not caveat the work as exploratory data science?

For example, if I've been totally sedentary for the whole day (and my feet are chilly+blue), a body temperature as low as 101F is unbearable. But if I've been actively moving around all day (and my feet are warm and pink), I only start getting uncomfortable at a body temperature around 103.5F-104F.

This also seems to correlate over a longer timespan re: exercise habits, consumption habits, sickness, etc.

Also IMO your body fat/water/lean muscle ratio may play a role. I once lost 5 kg due to Influenza A and all my sport achievements as well as sauna endurance were gone

I've got the opposite problem: saunas don't seem to be able to make me sweat anymore, so I'm looking for the hottest saunas I can find.

Edit: to put it into some numbers, per one study[1] Finnish sauna sessions were on average at 75.9°C with SD 9.9°C. If we assume normal distribution, that means that more than 97 % of sauna sessions are at < 95°C.

[1] https://pmc.ncbi.nlm.nih.gov/articles/PMC6262976/

Finland life expectancy for 2023 was 81.69.

Norway life expectancy for 2025 was 83.23.

Japan life expectancy for 2025 was 85.27.

Sumo wrestlers in Japan have a life expectancy between 60-65 years or so - significantly lower than the other japanese.

I am not saying that sauna has no positive effect at all, but I would reason that the number one risk factor is ... weight. And I'd also still say that exercise is correlated here, if only secondary, e. g. you may be able to maintain better bodily functions if you exercise, if you can avoid injury. I do not think that going into the sauna rather than e. g. light running for 5 to 10 minutes or so, is anywhere near on the same level.

Editorialized title is wrong. n=256

> In a typical Finnish sauna, the temperature of the air, the room, and the benches are above the dew point even when water is thrown on the hot stones and vaporized. Thus, they remain dry. In contrast, the sauna bathers are at about 60–80 °C (140–176 °F), which is below the dew point, so that water is condensed on the bathers' skin. This process releases heat and makes the steam feel hot.

> A sauna is a room or building designed as a place to experience dry or wet heat sessions or an establishment with one or more of these facilities.

https://en.wikipedia.org/wiki/Sauna

EDIT: I guess it depends on your definition of "humid". But 90C and regular water infusions are pretty common sauna conditions.

According to this company plus some sketchy math I just did, the relative humidity can swing between 15% and 40%: all over the place, but generally pretty dry. https://www.vaisala.com/en/blog/2024-12/can-you-handle-heat-...

For Claude we have the ever present "you are absolutely right" and this is like it's human mirror.

Something like TLDR; but meaning "uhg, written by AI".

1. https://www.scottsmitelli.com/articles/em-dash-tool/