anecdotal -but- it took me 6 months after covid for my breathing rate to go back to normal, and to be able to do consistent max our efforts of >190BPM for >5 seconds like previously
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.
We worked on a publication for NPJ biological timing and sleep
A few things that are surprising from the research
- sleep duration decreases by 4.4 minutes for each additional hour of daylight
- The effect didnt get much stronger at higher altitudes (even though daylight swings are much larger)
Plain English summary here
https://tryterra.co/research/terra-research-nature-publicati...