- Sauna recovery signals are real — we compared sauna and non-sauna days across 256 wearable users. Resting heart rate drops, activity is higher, and the effects hold even after controlling for exercise. The data backs what sauna believers have been saying for years.
- Nighttime heart rate drops ~3bpm on sauna days — even after controlling for higher activity levels, minimum heart rate falls on sauna days. That's a physiological recovery signal, not just a side effect of the workout that came before it.
- The effect is different for women — menstrual cycle phase matters. The heart rate drop only really kicks in during the luteal phase. During the follicular phase, it's there but small. Timing might matter more than we thought.
Sauna
Saunas Lower Your Heart Rate More Than Exercise
We analyzed 59,000+ daily records from 256 sauna users to quantify what saunas actually do to your body. Using paired t-tests on same-day wearable data, we found a clear recovery signal that lowers nighttime heart rate, higher activity, and distinct effects across males and females and menstrual cycle phases.
Rocio Mexia DiazAI and Health Researcher
March 26, 2026
Key takeaways
Saunas have been around since the primitive years in ancient Finland, and have always been considered to have a therapeutic effect[1]. Saunas are a hot, dry environment used to stimulate our cardiovascular system. During extreme heat exposure, our heart rate rises and our vessels dilate to increase the delivery of blood volume in order to protect the body[2].
This extra pressure on the heart is known to have long-term health benefits[3]. The heat exposure also promotes sweating and therefore the elimination of toxins, including those generated in the process of repairing small muscle tears after exercise[4]. It is for this reason that saunas are also considered great for recovery. All of this is no news, at the end of the day isn’t that what roman baths were built for? For recovery after battle[5]!
However, most studies have looked at the benefits of frequent sauna bathing and the impacts on long-term health. Motivated to understand the immediate physiological response to saunas, we looked at the same-day effects across ~59,000 daily records from 256 users.
Same-day Effects of Saunas
We used simple paired t-test evaluations to assess the immediate same-day effects of saunas.
Sauna days were associated with:
- Higher activity (longer activity time and greater distance)
- Higher maximum and average heart rate
That fits our intuition: many people sauna after a workout.
Sauna days also showed lower minimum heart rate compared to non‑sauna days. Importantly, this effect remains even after controlling for activity, which suggests the lower nighttime heart rate isn’t simply due to exercise. The difference between sauna and non-sauna days is on average 5% (3bpm) which is a noticeable physiological change.
These results were statistically robust (FDR‑corrected p < 0.05 and Cohen’s d > 0.2), supporting the idea that sauna use may be linked to better same‑day recovery.
Get the latest Terra Research reports and insights every week as soon as they're published.
By continuing, I agree to the Privacy Policy and Terms of Service.
Do Saunas Affect Women Differently?
Females showed larger activity increases on sauna days, which may reflect more consistent sauna use on workout days. However, females showed a smaller drop in minimum heart rate than males on sauna days.
As we looked at in previous blogs, the menstrual cycle can influence recovery and nighttime heart rate. For this reason, we evaluated sauna effects across the follicular and luteal phases and observed statistically higher activity and lower heart rate when women use the sauna on their luteal phase. In fact, heart rate at night was only meaningfully lower (Cohen d > 0.2) compared to non sauna days during the luteal phase. Or in other words, the benefits of saunas seem to appear only during the luteal phase…
What Does This All Mean?
Sauna use is part of a recovery‑oriented day. Sauna days are more active, which fits how people actually use saunas, often as a post‑workout routine. Yet even after accounting for activity, nighttime minimum heart rate is lower on sauna days, suggesting a physiological recovery signal beyond exercise alone.
Mechanistically, this pattern is consistent with known heat‑stress physiology: heart rate increases during sauna exposure, followed by recovery dynamics that can reflect increased parasympathetic influence during cooling [6][7] Within women, the strongest recovery signal in our dataset appears in the luteal phase, where the effect size crosses a meaningful threshold.
References
- UNESCO. Sauna culture in Finland. https://ich.unesco.org/en/RL/sauna-culture-in-finland-01596
- Ketelhut, S., & Ketelhut, R. G. (2019). The blood pressure and heart rate during sauna bath correspond to cardiac responses during submaximal dynamic exercise. Complementary therapies in medicine, 44, 218–222. https://doi.org/10.1016/j.ctim.2019.05.002
- Laukkanen T, Kunutsor SK, Khan H, et al. Sauna bathing is associated with reduced cardiovascular mortality and improves risk prediction in men and women: a prospective cohort study. BMC Medicine. 2018;16:219. (PMC: PMC6262976)
- Kuan, W. H., Chen, Y. L., & Liu, C. L. (2022). Excretion of Ni, Pb, Cu, As, and Hg in Sweat under Two Sweating Conditions. International journal of environmental research and public health, 19(7), 4323. https://doi.org/10.3390/ijerph19074323
- Marcussen, W. (2019, August 23). The Roman Baths in Bath- A Deep Dive into Britain’s Ancient History. World History Encyclopedia. **https://www.worldhistory.org/article/1427/the-roman-baths-in-bath--a-deep-dive-into-britains/**
- Laukkanen JA, Laukkanen T, Kunutsor SK. Cardiovascular and Other Health Benefits of Sauna Bathing: A Review of the Evidence. Mayo Clinic Proceedings. 2018;93(8):1111–1121. (PubMed: 30077204)
- (Study) Recovery from sauna bathing favorably modulates cardiac autonomic nervous system. 2019. (PubMed: 31331560)
Continue reading
Marathon Series
The Data Behind the London Marathon
While Sabastian Sawe was busy breaking the two-hour marathon barrier in London, the watches of 571 amateur runners told a quieter but equally fascinating story. GPS devices over-measured the course by 527 metres on average, faster runners ran with higher heart rates than the slower ones did, and Coros watches reported 47% more calories per kilometre than physiology actually supports.
April 27, 2026
London Data
Tube Strikes Made Londoners Active
During the April 2026 Tube strikes, Londoners adapted by cycling more—activity tripled, e-bike commutes rose 13%, and 19% of cyclists were entirely new to riding. Data analysis of 1,566 weekday rides revealed e-bike share jumped from 54.9% to 67.5%, with commutes often staying local. The strikes disrupted transit but highlighted Londoners' resilience in finding alternative ways to move.
April 24, 2026
Endurance Training
Running Is The Hardest Endurance Sport? Not so Fast
Which sport is hardest on the heart? We ran the question through hundreds of thousands of sessions across nine activity types. Running dominates minute-for-minute. It has the highest average, highest peak, highest sustained intensity. But its shorter sessions mean total cardiovascular load per outing often falls behind skiing or hiking. The shape of the heart rate curve tells a different story for every sport, and the weighting method you pick changes the answer.
April 23, 2026
Heat Exposure
Cold Plunging Might Make Your Biomarkers Worse
We analyzed large-scale wearable data to understand what cold exposure actually does to your body. Sporadic plunges act as a stressor, raising sleeping heart rate. But with consistent use, around 3 sessions every 14 days, the effect flips, improving recovery and sleep scores. We also found that women's responses differ significantly by menstrual cycle phase.
April 14, 2026
