8 ms·
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-
by kyriakosel 6mo ago
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.
- itsthecourier 6mo agohow does this reduction in heartbeat at night affect the body?
- croemer 6mo agoWhy didn't you put the methodology in the post? Also, which devices were used to record? How do you know people went to sauna?
- bluGill 6mo agoThe 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.
- testing22321 6mo agoI don’t think the TV in the sauna will have long term health outcomes.
- Filligree 6mo agoIt will certainly affect the health of the TV.
- kyriakosel 6mo agoAgreed - 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
- nsbk 6mo agoI 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.
- mminer237 6mo agoEdit: 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.
- rhyperior 6mo agoThat seems… misguided. Sources?
- gamerslexus 6mo agoIt 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 6mo agowe agree - but thats not that simple :)
- methyl 6mo ago> 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 6mo agoi was mostly refering to humidity/duration/temperature given that most devices do not report back these values
- austinthetaco 6mo agoI'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.
- gamerslexus 6mo agoWhat if they jumped between saunas? And with self-reporting, the more you ask I guess the less precise the result... Sensors, however...
- PaulDavisThe1st 6mo ago
- lccerina 6mo agoIf 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 6mo ago> 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.
- gjulianm 6mo agoUnless the effect they're measuring is that the wearable measures differently in sauna days.
- ranguna 6mo agoStrong conclusion that the hardware is precisely imperfect?
- lccerina 6mo agoNot 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
- jtanderson 6mo agoJust 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?
- Glemllksdf 6mo agoOr the sauna is a relaxing thing like a happy place and that reduces heart rate?
- gjulianm 6mo ago> 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 6mo ago> 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 6mo agoIt does, specially if the error bars from multiple measurements show higher precision than what would be expected.
- jampekka 6mo agoI 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 6mo ago> 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.
- al_borland 6mo agoHow 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 6mo agoDue 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.
- jampekka 6mo agoI'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
- lazyant 6mo agoWould a hot tub session (say at 100 - 105 F) be comparable or yield similar results?
- pstuart 6mo agoDr. 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=en https://x.com/foundmyfitness/status/1955294334535995850?lang...
- PaulDavisThe1st 6mo agoDavid 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 6mo agoBeing 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.
- Tor3 6mo agoAs 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)
- Noaidi 6mo agoJust 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 6mo agoIf 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 6mo agoYou think being n a Sauna is not a drug (effectively)?
- oidar 6mo agoAlso not controlled: Maybe on Sauna days they drank more water before bed? Or less alcohol?
- p1anecrazy 6mo agoAppreciate it as a regular sauna-goer. I am also struggling to wake up after sauna evenings and maybe you research explains why
- vinni2 6mo agoA 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 6mo agoYou are replying to a comment that said they log and control for time of activity.
- socalgal2 6mo ago> 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
- Tor3 6mo agoFrom 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 6mo ago> 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 6mo agoYes.. 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 6mo agoActually, 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 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 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.
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- alfiedotwtf 6mo agoTime 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.