[kyriakosel]

Author here. Methodology upfront because I'd ask the same things:

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.

[bluGill]

The most important thing you didn't measure: does this affect long term health in the same way exercise it known to. That is can I put a TV in my sauna and watch that for an hour every day instead of getting out and exercising - yet get the same better long term health outcomes?

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.

[nsbk]

I recently listened to a podcast about the benefits of sauna or deliberate heat exposure and the gist is that if you get your core temperature at about 39 degrees celsius your cardiovascular system is working comparably hard to light exercise.

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.

[shmel]

Not saying you are wrong, but I'd like to see some evidence on that. Just because your heart is pumping faster doesn't mean your cardio fitness is getting better. Otherwise we could all just snort cocaine and skip the gym. Alcohol does that too, anyone with a fitness tracker can check that.

[asdfman123]

Athletes already know the answer from years of cultural knowledge, research, and firsthand experience. No, it doesn't make your cardio fitness meaningfully better. If you did sauna training for years and then tried to ramp up for a marathon, you'd be hopelessly out of shape.

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.

[monkpit]

You’ve honed in specifically on VO2 but what about cardio health in general? Like light treadmill, not like a demanding marathon.

[asdfman123]

Cardio of course is short for "cardiovascular system," which consists of a whole lot of moving parts. Saunas improve some parts of it but certainly not all of it.

Will fixing up your radiator fix your car? Maybe, if the radiator was the problem, but there's a lot of other stuff inside a car to work on, too.

Your body evolved under the expectation that it would be stressed in numerous different ways, but those stressors can all be avoided in the modern era. If you want to most reliably recreate those stresses you need to do cardio and resistance training.

[barnabyjones]

Without exercise, you won't burn ATP and thus won't increase mitochondrial count.

[nradov]

A light treadmill session won't do much to improve your cardiovascular system health either. I mean it's better than nothing but don't expect too much.

[xtracto]

Moreover, I'm from a very hot and humid tropical region. Its normal to ne 40°C with 80% humidity there. And you dont see people having better health or longevity (Yucatan peninsula) .

[smileysteve]

40° internal body temperature is not the same as 40° weather.

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.

[Tor3]

Right, it's just that a sauna at 60 degrees is not warm, it's cold. Take a shower, go into the sauna at 60 degrees C, and it'll feel cold. Nothing happens in a sauna until you're getting near 80, and it's much better if you go somewhat higher (90 or more for active users). 60 is when a sauna will be closed off in public baths because there's a technical problem somewhere.

[actionfromafar]

But that would be like exercise all the time which may not be optimal. (Not saying the theory holds that sauna equals exercise, but if it does, sauna all the time may not be great. Plus, there may be other confounding factors with living in various locations.)

[Insanity]

The great but not super healthy Mexican diet might offset the potential heat exposure benefits! Although I’m basing that on the diet of my Monterrey-based in-laws, not sure how different Yucatan is.

[xtracto]

LOL, Monterrey diet is healthy compared to the diet in the Yucatan peninsula.

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.

[mminer237]

Edit: I posted this accidentally when editing without noticing. Hypertrophy isn't necessarily a bad thing. I thought I was discarding the comment cuz I realized I was out of my depth. whoops

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.

[kiritanpo]

Cardiac hypertrophy is not necessarily a bad thing, it can be the result of positive adaptation, such as exercising.

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.

[kazga]

Not true. You can't really train your muscles to use less oxygen for the same energy output (what "oxygen-efficiency" would imply). You rather increase their capacity to take up oxygen from the blood and burn it. They will use more oxygen to output more energy.

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.

[bluGill]

You kind of can - the muscles can use aerobic or anaerobic processes. When you develop brute strength you are training those anaerobic processes. That isn't what OP was talking about, and overall it is much less energy efficient, but it does produce a large burst of energy when needed and you can train your muscles that way.

[gf000]

I would assume that another factor is that the technique for a given exercise on the other hand can be improved, and that can help with decreasing the necessary energy - would that be a correct statement? And as a follow up, depending on activity type this may or may not be significant?

[frocodillo]

Yes, definitely. Technique is partly about efficient mechanical movement, sending the various parts of your body in the right direction(s) and not waste effort on movement that doesn’t contribute to propelling you forwards. But for endurance sports, it’s really about minimizing energy cost at a given speed. To use running as an example, you can improve biomechanical efficiency through better timing, correct loading of tendons, tendon stiffness, elastic energy use, and more.

[SJMG]

This is terribly uniformed. Do not listen to this.

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.

[adrianN]

Wikipedia says Athlete's heart is benign: https://en.wikipedia.org/wiki/Athletic_heart_syndrome

[groundzeros2015]

This is why I hate health science. Informed people can have the same information and come to opposite conclusions. The entire field is made up of contradictory explanations and principles, to the extent that it’s unknowable what’s true or not.

[robrenaud]

The flat earthers are why I hate astronomy.

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.

[groundzeros2015]

It’s not really about this particular claim. It’s that I can read a comment that has a reasonable chain of logic and I don’t know if it’s true. This topic is just not easily studied and theories are hard to falsify.

[groundzeros2015]

Claims about flat earth are falsifiable with at-home experiment.

[rhyperior]

That seems… misguided.

Sources?

[jagged-chisel]

Your heart is also a muscle.

[PaulDavisThe1st]

... about the size of your fist. Keep loving, keep fighting.

(attributed to Bertolt Brecht)

[bluecalm]

For endurance training the main benefit of heat training is raising blood volume. Lungs are not a limiter. Developing stroke volume I imagine requires much higher intensity but that's just a wild guess based on my limited understanding of physiology.

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.

[PaulDavisThe1st]

and specifically, to be clear, plasma volume.

[pegasus]

Since you mention the TV, it seems there's a big factor missing in both the article and the discussion here. Namely, that sauna time is for many people the only time they ever take to be in silence, without the countless distractions otherwise bombarding our nervous systems. I.e. it's basically a form of informal meditation, which is known to have a lot of benefic impacts on body and mind. So maybe skip the TV part?...

[willnz]

I recently got an outdoor sauna at home, and that's definitely a key benefit ...sitting in silence without any devices, no smart phones, watches or music for at least 15-20 minutes.

[kyriakosel]

Agreed - one is muscular/metabolic demand, the other (sauna) is thermoregulation.

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

[nyjah]

Zero shot you'd make it an hour in a proper sauna for an hour. People have this idea that saunas are always enjoyable. I sauna daily, and its nice up to a point. For me thats like 10-12mins in. From then on, its tough.

[tuukkah]

When it doesn't feel enjoyable anymore, you're supposed to get out of the sauna and cool down - preferably in a lake. Then repeat as many times as you like.

[Tor3]

Yep. A problem with public saunas (outside Finland, at least) is that they lower the temperature (to, say below 80) in the misguided belief that this will make it easier for more people to stay longer inside.. which is the wrong way to use a sauna. A sauna should be hot enough that you'll go out when it starts feeling too hot (or hard), not so cold that you'll go outside when you're getting bored. With a hot sauna small children can leave after one or two minutes, some people leave after five, others after ten, and in any case go outside and cool down with a showre (or a lake..), then go back in.

[iammjm]

I go for 15 min sessions at 90 Celsius and the first 10 mins are ok, the last 5 are tough, like I have to control my breath to hang in there

[httpsterio]

Huh what? I can easily sit in a sauna for an hour without breaks as long as it has some type of ventilation.

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.

[Tor3]

At which temperature?

[httpsterio]

The one event where we say and drank, it was about 90 deg celcius and lots of water on the stove (löyly).

[bluGill]

I generally make it about 30 seconds in a sauna (I rarely even bother trying when I have access). Should I tough it out for 10-12 like you? Should you be toughing it out for the full hour I suggested (a random time I pulled out of my head)? Or is this all nonsense and I'm just fine ignoring the whole thing?

[Tor3]

Don't "tough it out" in a sauna. Stay until it feels uncomfortable, or, if you're not sure, keep track of your heart rate and get out if it increases too much. You'll get used to the sauna after frequenting it for some time, which usually results in being able to stay longer before it feels uncomfortable. Your ability to sweat will improve, for example, including being able to sweat on body parts where you may initially be unable to (it took a long time before my wife's calves "learned" to sweat, for example).

[MyHonestOpinon]

IMHO. 10-20 mins is good enough. I wouldn't try to stay for an hour.

If 10mins feels too much, do less.

[theshrike79]

30 seconds? What kind of torture room of a sauna are you going in?

[marssaxman]

Heat tolerance varies. For some of us, all saunas feel like torture rooms!

[trklausss]

You are clearly not Finn (/s)

[nyjah]

lol, this is true. Wish I could tolerate it longer like a proper Finn. I’ll go 25 mins occasionally but mostly I do 15 mins, break, another 5-10 mins..

[jampekka]

Finns don't do it that long. It's typically 5-15 minutes per sitting, with 2-4 sittings per session.

[nyjah]

My answer was sarcastic back, fwiw, I follow the 'proper' finnish ritual or similar...

[lfuller]

This feels like a false dichotomy. Even if sauna doesn't impact long term health in a way that can replace exercise, that doesn't mean that it doesn't improve health.

[HelloMcFly]

> That is this improves a marker for good health without improving health

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.

[gjulianm]

Much of that body of evidence relies on self-reported and self-assigned sauna usage rather than actual randomized trials, and also the papers show massive risk reductions that do not really fit with the country-level data (e.g., if saunas are that good for cardiovascular health and finns use them that much, why do they have similar rates of CV disease as neighboring countries that don't use that much sauna?)

[HelloMcFly]

Much of it is, sure, but certainly not all of it! On your comparison to Sweden, be cautious! Finns generally have a higher risk and incidence of cardiovascular disease compared to native Swedes - in fact, they have some of the highest risk in the world!

Research from Earric Lee and/or Jari Laukkanen from this past decade will have clinical trials with controlled groups rather than just long-term population tracking. There are within-Finland studies comparing high-risk Finns who use the sauna 4 to 7 times a week against high-risk Finns who use it only once a week, showing a clear effect (https://pubmed.ncbi.nlm.nih.gov/25705824/). Here is a non-randomized experiment showing a dose-response (https://pubmed.ncbi.nlm.nih.gov/29048215/).

Those are just indications of information available. I would also argue that while of course randomized experiments are ideal, it is a mistake to dismiss all other forms of evidence so readily, especially with such preponderance of it.

[gjulianm]

The first study shows a 0.37 hazard ratio for frequent sauna use. That’s better than a lot of the HRs reported for physical activity. It’s even better than not smoking. It just doesn’t really fit.

Also, the fact that there are practically no sauna related studies outside of Nordic populations is suspicious too. I bet that with those hazards ratios a lot of people have tried to study the effects more, it’s suspicious there’s practically nothing out there.

[HelloMcFly]

There's more research for you to explore if you're interested, but you sound more closed than curious if I'm honest. Maybe looking more into it will change your mind, maybe not!

[jjallen]

My current guess is that you get much or most of the benefits, but not all (by both value and number). If you look at the actual changes in the body during both of these activities, most are the same as exercise, but not all.

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.

[ricardobayes]

My take is probably too nuanced here, but the reality is that we don't know. People living in areas with longevity (blue zones), didn't really excercise (as in sports) or take multivitamins. For all we know, it might even come out that regular, gym-style excercise is even worse for longevity.

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.

[somebodythere]

There is some evidence suggesting that "blue zones" are largely about pension fraud. https://fortune.com/europe/2024/12/14/are-blue-zones-myth-ex...

[jeffbee]

You're saying that crime leads to longevity? Big if true.

[taeric]

I think the claim is more that if you provide financial support for X without solid record keeping to verify X, expect that you will get more self reported people in that description.

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

[yxhuvud]

No, he is saying bad record keeping means misreporting identity has a bigger chance of happening.

[projektfu]

Fraud leads to people officially living beyond their natural death, yes.

[RankingMember]

> I'm not arguing that excercise is unhealthy, it's just that its contribution to eventual longevity, is currently unknown

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/

[ricardobayes]

Thank you for the resources.

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?

[standardly]

The equation is not simply "exercise" -> "live longer" via some unknown correlation.

Exercise increases cardiovascular health, mental well-being, etc. It should be pretty obvious that someone with less risk of heart failure will live longer on average (considering the #1 cause of death in the US...) I don't think you need to factor for every possible life circumstance to deduce that known-healthy activities improve longevity

[brushfoot]

> People living in areas with longevity (blue zones), didn't really excercise (as in sports)

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.

[trklausss]

For all we know, there is a link between cardiac/circulatory problems and arteriosclerosis (that is, loss of elasticity of the vessels).

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.

[bluGill]

Problem is sauna use and genetic factors corrolate too strongly to make any conclusion to the broader population. If you live in/near Finland you likely sauna often, as have all your ancestors for thousands of years. If you don't live there both are false. Thus we can't know if Sauna is helpful for the general population who isn't of a Finish background.

[shevy-java]

Japan has a +4 years lead of life expectancy over Finland; Norway almost +3 years on Finland. I am not saying this is conclusive per se, but to me the sauna-people-live-forever is not backed up by the data. I would instead reason that, e. g. weight correlates a lot more here.

[smileysteve]

Your comparison reads to suggest that Japan doesn't have Onsen culture or that sauna does not exist in Norway.

That's to say, many cultures from around the globe have developed similar activities that heat the body.

[Tor3]

Saunas are common in Norway, even if not to the same degree as in Finland. The reason Finland has had a lower life expectancy than Norway is believed to be due to the difference in diet (cardivascular issues). Note that the diet in Finland has changed quite a lot over the last decades and these differences will presumably level out, statistically.

[bluGill]

Nobody is claiming they live forever. The claim is sauna use increases lifespan. There are other factors than just sauna use in lifespan though. The question is would the Japanese live even longer if they were using a sauna?

[gonzalohm]

A lot of Mediterranean countries also have high life expectancy and are the opposite of a sauna culture.

[kyriakosel]

Cyprus summers are like 45C and its almost like a sauna :)

[GordonS]

Maybe eating a lot of fish, rather than meat, has an impact too.

[bwestergard]

The "blue-zone" studies are flawed, so we shouldn't infer too much from lifestyle generalizations about people in them.

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

[idiotsecant]

Saunas have not been around longer than exercise.

[z3t4]

I think that if you have one hour or more of free time and live in an area where you have easy access to a sauna, that would result in significant better health on it's own. Even if you choose to not use the sauna.

[testing22321]

I don’t think the TV in the sauna will have long term health outcomes.

[Filligree]

It will certainly affect the health of the TV.

[sonink]

I looked into Saunas in detail sometime back as a replacement/complement to exercise. There is a lot of research out there which says Saunas are as beneficial - but at the end of it I reached a similar conclusion - exercise is just better understood, so no point experimenting when something can go wrong.

[havella]

This question is dead from the beginning. Exercise is good for the heart, the muscles, metabolism. You do need muscle contractions above certain level of intensity and duration for this to happen.

[Jtarii]

A sauna will do nothing for muscular-skeletal health.

[derektank]

That seems like a very strong statement. Isn’t there evidence that Heat Shock Proteins are produced in response to time in the sauna, which have beneficial effects on muscle growth and repair?

[Jtarii]

>which have beneficial effects on muscle growth and repair?

Repair from what?

[derektank]

Mechanical stress, i.e. exercise

[iso1631]

If you're watching TV in a sauna you haven't turned the sauna on.

[tester756]

One hour in sauna? :O

[bluGill]

A random time I pulled out of my head. If this is real the next question is what is the optimal time. (also temperature and humidity levels)

[jmdeon]

I'm being slightly snarky, but good luck watching a TV if you're doing an intense/valuable sauna session.

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.

[throwaway-blaze]

Even more likely is those using saunas and tracking metrics with wearables are self-selected to be healthier/more active/etc. Correlation and causation...

[Tor3]

From the article: "..promotes sweating and therefore the elimination of toxins,.."

That false statement really makes me unsure about the quality of the article. And I'm saying this as someone who uses sauna daily, when possible (I have one at home, and I grew up with saunas).

"De-toxification" by sweating is a myth. Sweat glands are very simple organs (think salt on one side, which results in pressure, i.e. osmosis) and can't do anything of the sort. You'll be much better off peeing.

Saunas probably have good health effects. I'm certainly happy as a sauna user. But there's no de-toxification in this.

[Ey7NFZ3P0nzAe]

> You'll be much better off peeing.

Actually, the rate of blood filtration by kidney glomerulae is pretty constant and independent of the amount of urine in your bladder. Except if you overflow to the point of drowning your kidney of course.

[Tor3]

Yes.. my point was only that to get rid of more than water you're better peeing than sweating. In any case, the liver is the only toxin-handling organ in the body (and a complex one).

[majkinetor]

Actually, this is why one shouldn't believe HN comments about medicine. The quality is this low.

This is simply laughable and very easily checked out. Here is one random study:

https://link.springer.com/article/10.1007/s00244-010-9611-5

> Many toxic elements appeared to be preferentially excreted through sweat

Here is another quick one

https://onlinelibrary.wiley.com/doi/full/10.1155/2012/184745

> Sweating deserves consideration for toxic element detoxification.

Its even observable directly - just go next to the heavy smoker during exercise.

While it might be less than dedicated organs, the skin is the largest organ by far.

Ofc, the topic is debated like every single thing in medicine, but calling it a myth is nonsence.

[Tor3]

First, you don't need to go through any kind of "detox" regime. That's myth 1. The liver is where natural detoxing occurs, and the kidneys filters the rest. Don't focus on detox, instead focus on keeping your liver healthy (alcohol, fat..) Secondly, the sweat glands don't have any kind of functionality outside of pressing water out of the body. The water may happen (depending on how small the molecule is) to bring other stuff with it, but that's hardly "detox". Just pee instead! The whole "detox" trend is just that - a trend, a meme. Focus on other elements of health instead, eat healthy, and don't interrupt that with some tiktok "detox" scheme.

[majkinetor]

Looking further into it, it seems that we simply don't know much about it, but as I pointed out earlier there is measurable waste disposal. You can imagine that precise knowledge is difficult to measure as it probably must be done separately by each waste product.

This review from 2019 states:

> However, the effectiveness of sweat glands as an excretory organ for homeostatic purposes is currently unclear as there are no comprehensive reviews on this topic.

https://www.tandfonline.com/doi/full/10.1080/23328940.2019.1...

It also mentions stuff similar to the things you wrote:

> The role of sweating to eliminate waste products and toxicants seems to be minor compared with other avenues of excretion via the kidneys and gastrointestinal tract; as eccrine glands do not adapt to increase excretion rates either via concentrating sweat or increasing overall sweating rate. Studies suggesting a larger role of sweat glands in clearing waste products or toxicants from the body may be an artifact of methodological issues rather than evidence for selective transport.

However, the problem here is that normal sweating is taken into account, but discussion here is about sauna/exercise in which case sweating waste products approach kidney flux rates (which is why people do those things to detoxify) - human can easily sweat 1.5L per hour in that case, which is comparable to daily pee volume, and there are measurements regarding sweat composition that show comparable waste excretion:

    | Substance | Sweat (1h) | Urine (24h) |
    | --------- | ---------- | ----------- |
    | Urea      | ~0.3–1.2 g | ~20–30 g    |
    | Lead      | ~5–50 µg   | ~10–50 µg   |
    | Arsenic   | ~10–60 µg  | ~20–200 µg  |

So, I wouldn't dismiss based on current knowledge, and besides, this has no practical importance even if so, as sauna has the number of other measurable benefits.

Otherwise, I agree with you regarding focus - liver should be treated as sanctuary.

[gjulianm]

> Effect survives controlling for activity level.

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?

[jampekka]

> What wearables were used? These devices don't usually have enough precision to reliably detect ~3bpm changes.

For large sample averages this doesn't really matter.

[gjulianm]

It does, specially if the error bars from multiple measurements show higher precision than what would be expected.

[jampekka]

I don't understand what you mean by that.

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.

[gjulianm]

> of estimate of mean

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.

[jampekka]

> 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.

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.

[lccerina]

If this was a peer-reviewed paper, it won't pass.

- 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?

[joelthelion]

> Is the wearable accurate enough to be sure that 3bpm is not a measurement fluke

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.

[lccerina]

Not at all, if you have a lot of data coming from imperfect hardware (which can mean both a fixed bias and unknown variance), and you don't know the variance for plenty of practical reasons, you are left with a result that is statistically significant, but wrong

[gjulianm]

Unless the effect they're measuring is that the wearable measures differently in sauna days.

[ranguna]

Strong conclusion that the hardware is precisely imperfect?

[jampekka]

I'd like to see a bit more detailed methods.

- 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

[gamerslexus]

It would be very interesting if lowering night heart rate only happens with certain sauna type.

> 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...

[kyriakosel]

we agree - but thats not that simple :)

[methyl]

> who logged sauna sessions via connected apps

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?

[kyriakosel]

i was mostly refering to humidity/duration/temperature given that most devices do not report back these values

[austinthetaco]

I'm equally confused as the other person above. Why not just ask participants to report what type of sauna they used? Sure humidity/duration/temp would be awesome to have, but at the very minimum knowing if a dry sauna would get the same results as a traditional steam sauna.

[PaulDavisThe1st]

There's quite a wide range of variation between "full dry" (no added humidity whatsoever) and "full steam" (an actual steam room, rather than a sauna). Just asking people was it dry or was it humid won't capture much of that variation. I have a steam room at home and have been a near-life-long lover of them - they are wildly different than a sauna. But I'd still rank a sauna where someone had dumped 1L or so of water over the heater to be "humid", and consider it also very different from a totally dry sauna.

[gamerslexus]

What if they jumped between saunas? And with self-reporting, the more you ask I guess the less precise the result... Sensors, however...

[alfiedotwtf]

Time of day could make a huge difference - if they did this close to sleeping, their body would be hot while winding down, and so would use less layers. If they fell asleep with only a sheet compared to multi-layered thick covers, their temperature would be lower for the whole night.

I know n=1, but when I started sleeping with only a sheet, my heart rate was at my lowest - my Apple Watch would ping multiple times a night because it was below 40.

Can’t comment about being cold anymore because these days I sleep on top of a spiked mat which makes my back feel like it’s on fire for the whole night.

[al_borland]

How would this play out over time? Will sauna see a 3bpm drop below baseline on days it’s used, while keeping the same baseline?

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.

[verst]

Due to lots of long distance running my rest heart rate is below 40. I am highly skeptical I would experience a 3bpm lower heart rate after sauna. Maybe this benefit applies after infrequent activity or less intense activity only.

[jtanderson]

Just as a discussion point: how do you think these effects would translate (if at all) to regularly practicing hot yoga, say around 100-105F? Intuitively, it would combine the effort + recovery, but probably not enough time elapsed in the same session for the sweat benefit during muscle repair?

[oidar]

Also not controlled: Maybe on Sauna days they drank more water before bed? Or less alcohol?

[lazyant]

Would a hot tub session (say at 100 - 105 F) be comparable or yield similar results?

[Tor3]

As someone with access to both Japanese ofuro and sauna, they are quite different in some respects. And similar in others. One thing which a sauna could do for me when the ofuro could not, was to fix a problem I had with coughing. Something which plauged me for a long time, and which the doctors couldn't find any reason for, but I had such painful daily coughs that it really bothered me. Couldn't sleep on my back either. Then I noticed that if I used the sauna daily, and carefully breathed hot air, the symptoms lessened. And after going for the daily sauna regime (instead of occasionally) for some time, the coughing problem I had for years finally disappeared. The hot baths did nothing for this (but was good for other things, e.g. muscle pains. And essential for being able to sleep at cold winter nights in non-insulated Japanese homes.. heating up the body with a very long very hot bath does wonders)

[pstuart]

Dr. Rhonda Patrick has been beating the drum for saunas for a long time, and she's reported that a hot tub can be equivalent or better: https://x.com/foundmyfitness/status/1955294334535995850?lang...

[PaulDavisThe1st]

David Roche, a notable running coach (and runner), and his co-coaching wife (and runner) Dr. Megan Roche (MD/PhD) seem to think that hot tubs need to be at least 106F to generate much of a heat shock response, which is normally what one is looking for in the context of post-exercise heat exposure. I should note, however, that they are mostly reading the same research papers as everybody else, not doing primary studies themselves.

[pstuart]

Being that the recommended max temperature for hot tubs is 104F that could be an issue. Hot tubs are definitely more pleasurable than saunas, and if it comes "close enough" I'd be fine with that.

[socalgal2]

> Sauna days are more active, which fits how people actually use saunas, often as a post‑workout routine

WAT? As far I as know there is no such connection between workouts and saunas in Finland, nor in Japan

[vinni2]

A lot of people go to sauna after workout. I rarely go to sauna without workout so not sure if the combination is helping me or exercise or the sauna. How to control for that?

[marcosdumay]

You are replying to a comment that said they log and control for time of activity.

[p1anecrazy]

Appreciate it as a regular sauna-goer. I am also struggling to wake up after sauna evenings and maybe you research explains why

[Glemllksdf]

Or the sauna is a relaxing thing like a happy place and that reduces heart rate?

[itsthecourier]

how does this reduction in heartbeat at night affect the body?

[Noaidi]

Just because your heart rate is lower does it mean you’re any healthier however. This is just ridiculous measurement it means nothing.

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?

[PaulDavisThe1st]

If your resting heart rate is lower without any drugs, it indicates that your heart can cause sufficient oxygen to be delivered to your organs with less effort than if it were higher. That can be caused by a variety of things (including stroke volume, capillary dilation and general obstruction (or not) of blood vessels). These are all good proxies for general health.

Drugs lowering your resting heart rate do not indicate this in the same way.

[Noaidi]

You think being n a Sauna is not a drug (effectively)?

[croemer]

Why didn't you put the methodology in the post? Also, which devices were used to record? How do you know people went to sauna?