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.
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.
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.
I can tell you wrote the article with ChatGPT. I’m out as soon as I pick up the smell. I don’t dislike the usage of AI, I just don’t trust. It if you haven’t written it yourself.
> Motivated to understand the immediate physiological response to saunas, we looked at the same-day effects across ~59,000 daily records from 256 users.
n= traditionally refers to the number of participants, not the number of data points.
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.
N=1, but I started rowing (indoor, on an erg) an hour a day -- not hard, generally 120-140 bpm -- every day starting February 28, after rowing inconsistently for a year or more before that. My resting (not sleep) pulse has dropped by 10% over the past ~7 weeks, from 60 to 54.
General advice: citing absolute HR numbers is pretty meaningless for a broad audience, because they are not intepretable. Express them as percentages of your current max HR to be meaningful to others.
Anecdotal, of course, but the biggest change I ever made in my life was right before bed: take a screaming hot shower with dim lighting. I'd say 95% of the time, I get in bed and just pass out and have no real memory of time passing before falling asleep.
Not to be glib, but being dead lowers your night time heart rate more then exercise as well.
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.
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.
Seems to me what we now know about neural networks, we should maybe weighted sum of inputs, that fire off the desired output. The human body/brain process all kinds of stimulus at once, and might only react to a combination of inputs.
I know that for myself exercise increases my resting heart rate in the short term. It only decreases after a day or two, sometimes more depending on how fatigued I am.
I thought that was common, with recovery times obviously decreasing the fitter ones gets.
Can anyone suggest why after covid I can't do Finnish sauna anymore? Prior to that I used to do 1-2x a week a sequence of 5x(10 minutes in sauna + 5 minutes cold water immersion + 10 minutes rest) which was absolutely great for both stress reduction and blood flow. Now if I do 5 minutes in sauna I feel like my skin was burning and I am about to die, and I need to recover for 1 hour from that to be able to just walk away from sauna.
Is the stove radiating too much heat? You want it to heat the air, not fry your skin! I once got a stove glowing hot because I had failed to get a high enough temperature going on the day before. It was the first time I tried a wood fired sauna myself. As long as the stove was glowing hot, I just couldn't enjoy the sauna.
A delta of 3 bpm on sauna days corresponds to around 4% delta if the baseline is 72 bpm. I've gone from a resting heart rate over a 7-day average of 64 bpm to 58 bpm by jumping 15 min. of rope a day, 4 times a week. I've lost weight, body fat, and I feel like my body is more efficient with corresponding lower heart rates throughout my active day. I like saunas for recovery and aches, they put me in a relaxed state after, and I believe the dilation is flushing my system. Like anything else, moderation. Perhaps I will add sauna to my weekly routine 1x per week or less.
This would not pass peer review for a journal as written.
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?
I try to do 180 minutes a week of cardio. Mostly Zone 2. Biking, elliptical, tKD. But once in a while my legs feel too tired, so I complete my weekly minutes going to the steam room. It makes sense to me since it raises your heart rate.
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.
FWIW, most of the studies on improvements in blood plasma volume from passive heat are usually done on saunas with temperatures > 150 degrees F (60 C). Steam rooms usually only get up to 120F (~49C) even though the humidity probably makes it feel warmer.
Potentially, but likely much less effective and less studied, and you likely need longer sessions for effective dosage.
Most of the studies I've seen on improvements in blood plasma volume from passive heat are usually done with sessions in saunas with temperatures > 150 degrees F (60 C). Steam rooms usually only get up to 120F (~49C) even though the humidity probably makes it feel warmer.
Copying and pasting some of my reply to another comment above
TLDR: regular sauna seems to have no effect on my resting HR. Extended high HR cardio definitely does.
I became a huge fan of sauna time (15-20 minutes at approx 175F/80C... I would prefer a bit warmer, but I had no control). It was like shower time, or meditation time (which I never took time for). Great thinking time. I'd use it after a workout, but I would also use it on days without a workout.
I've been tracking some stats via my Garmin watch for a few years, and I've identified some patterns - particularly regarding resting HR.
The most significant reducer of resting heart rate for me is running (5k). Periods where my training includes regular 5k runs cause my resting HR to drop by 5-8 bpm.
Most of my training is resistance, although in the last 6 months I've added in a lot more cardio. Stairs and rowing do not seem to noticeably reduce my resting HR. Running definitely does. But to be fair, maybe it's not the running but rather the active HR I'm sustaining. Despite trying to stay in the aerobic zone, running always pushes me to zones 4 and 5. So 50% of my 30 minutes of exercise will be in my max zone. With stairs or rowing, I can keep my HR in aerobic and threshold.
Some stats:
When I'm off my fitness routine, living life as a typical person, my resting HR is 65. When I'm on a resistance fitness routine, my resting HR is 58. When I'm also running, the rHR is 51.
If I eat a heavy meal too close to bedtime, my rHR is +15. If I drink a lot of alcohol before sleeping, rHR can be +20! Food + alcohol = WTF. Probably not good.
39 comments
[ 12.0 ms ] story [ 68.5 ms ] threadData: 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.
WAT? As far I as know there is no such connection between workouts and saunas in Finland, nor in Japan
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.
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.
Editorialized title is wrong. n=256
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.
It increases blood flow in your legs, which lowers your blood pressure -> lights out faster.
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.
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.
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?
https://cptsd.sites.umassd.edu/bryan-johnson-and-microplasti...
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.
Most of the studies I've seen on improvements in blood plasma volume from passive heat are usually done with sessions in saunas with temperatures > 150 degrees F (60 C). Steam rooms usually only get up to 120F (~49C) even though the humidity probably makes it feel warmer.
Copying and pasting some of my reply to another comment above
TLDR: regular sauna seems to have no effect on my resting HR. Extended high HR cardio definitely does.
I became a huge fan of sauna time (15-20 minutes at approx 175F/80C... I would prefer a bit warmer, but I had no control). It was like shower time, or meditation time (which I never took time for). Great thinking time. I'd use it after a workout, but I would also use it on days without a workout.
I've been tracking some stats via my Garmin watch for a few years, and I've identified some patterns - particularly regarding resting HR.
The most significant reducer of resting heart rate for me is running (5k). Periods where my training includes regular 5k runs cause my resting HR to drop by 5-8 bpm.
Most of my training is resistance, although in the last 6 months I've added in a lot more cardio. Stairs and rowing do not seem to noticeably reduce my resting HR. Running definitely does. But to be fair, maybe it's not the running but rather the active HR I'm sustaining. Despite trying to stay in the aerobic zone, running always pushes me to zones 4 and 5. So 50% of my 30 minutes of exercise will be in my max zone. With stairs or rowing, I can keep my HR in aerobic and threshold.
Some stats:
When I'm off my fitness routine, living life as a typical person, my resting HR is 65. When I'm on a resistance fitness routine, my resting HR is 58. When I'm also running, the rHR is 51.
If I eat a heavy meal too close to bedtime, my rHR is +15. If I drink a lot of alcohol before sleeping, rHR can be +20! Food + alcohol = WTF. Probably not good.