alt Hacker NewsAuthor 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.
Replies
> 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?
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?
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
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.
Would a hot tub session (say at 100 - 105 F) be comparable or yield similar results?
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?
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...
Or the sauna is a relaxing thing like a happy place and that reduces heart rate?
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?
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?
how does this reduction in heartbeat at night affect the body?
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.