- Sporadic cold plunges act as a stressor, not a recovery tool — they raise your sleeping heart rate by ~1 bpm, triggering a sympathetic response as your body reacts to the rapid temperature drop.
- The stress-to-adaptation crossover sits at around 3 sessions per 14 days — below that threshold, you're just spiking sympathetic activity. Above it, cold exposure starts to improve recovery scores, sleep scores, and resting heart rate.
- Women's adaptive response differs by cycle phase — during the luteal phase, cold exposure raises sleep heart rate even with frequent use, suggesting the hormonal environment blunts the adaptation seen in other contexts.
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.
Rocio Mexia DiazAI and Health Researcher
April 14, 2026
Key takeaways
Plunging into freezing water is becoming more common recently, because its thought to have a positive effect on our recovery, resilience and overall health. Extreme cold conditions are known to impact our physiology and metabolism.
Inuit people can survive temperatures up to -31°C in the Arctic. The reason? That’s a high concentration of brown fat (brown adipose tissue)[1]. Brown fat is largely different from white fat, who’s high concentrations are associated with diabetes and cardiovascular disease.
White fat can turn into brown fat through exposure to cold temperatures; brown fat burns more sugars and calories which increases our metabolism but in turn also increases our calorie demands[2].
There is little agreement across studies on protocols, methods and effects across genders so, motivated to understand the benefits of cold exposure in large scale wearable data, we analyzed over 170,000 nights of data from 577 users.
Benefits (or risks) of Cold Plunging
We analyzed the physiological response to cold plunging using mixed-effects models that accounted for repeated measurements within each user and, in adjusted models, training load.
Cold plunges turn out to be a substantial stressor on the body. The rapid drop in temperature likely triggers an adrenergic response as the body shifts into a more sympathetic-dominant state[3]. Sporadic cold exposure sessions raise sleeping heart rate by around 1 bpm (p < 0.05). The wide error bars suggest that the impact is moderated by different user habits, such as the timing, intensity, and duration of the exposure.
A plunge close to bedtime, for example, may have a stronger effect than earlier cold exposure, and some users may also be logging milder exposures such as cold showers.
The real benefit seems to come with consistency. Using an interaction model, we found that each +1 SD increase in recent cold-use density was associated with an additional -0.58 bpm shift in sleep minimum heart rate in the activity-adjusted model.
In the unadjusted model, the shift was even larger at -1.05 bpm per +1 SD. The fitted curve crossed the null around 3 sessions in the previous 14 days, suggesting that this is the point where cold exposure stops looking predominantly stressful and starts looking adaptive.
We used this threshold in further analyses. Once users were in this adapted state, cold exposure days were associated with:
- +1.33 higher recovery score (p=0.015)
- +1.01 higher sleep score (p=0.057
- -0.97 bpm daily heart rate (p=0.066)
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.
Cooling Off After the Sauna
As we found in the previous blog, sauna use has a clearer same-day cardiovascular effect, especially on daily minimum heart rate. Cold exposure, by contrast, seems to behave more like an intervention that requires adaptation before benefits emerge. In other words, sauna appears to act on a shorter timescale than cold exposure, and the two are not necessarily best captured by exactly the same metrics.
To summarize the broader pattern, we created an exploratory recovery-oriented composite by averaging standardized recovery score, sleep score, and sign-flipped minimum and average heart rate, so that higher values reflected a more recovery-like physiological state overall.
In adapted users, all three conditions looked better than no exposure, but the highest composite value was seen on days with both sauna and cold exposure. Sauna-only was very close, suggesting that sauna may be the main driver of this effect, with cold adding only a small extra contribution.
Women’s Effects
Women have a naturally higher percentage of brown fat than male[4], and most studies have been conducted on males. Given the benefit of cold exposure is transforming white fat into brown fat, there surely is a different effect on a population with greater content of brown fat!
Paired within-user analysis showed that, on average, frequent cold exposure during the luteal phase was associated with an increase in sleep average heart rate of +1.68 bpm (d=0.62, p=0.002) rather than an adapted drop in heart rate. This comparison was made within women who frequently used cold exposure, comparing exposure and non-exposure days specifically during the luteal phase.
By contrast, the follicular phase showed no meaningful difference in sleep average heart rate (-0.06 bpm, p=0.92).
Conclusion
When cold plunges are used sporadically, they look like a stressor and raise sleep HR. When used repeatedly, they become associated with better recovery, better sleep score, and a progressively more favorable nighttime HR response.
Sauna appears to have the clearest immediate cardiovascular effect, while cold exposure seems to require repeated use before benefits emerge. Adaptation is key: Inuits did not wake up one day and suddenly tolerate the cold.
References
- Steph. (2016, December 24). Cold tolerance among Inuit may come from extinct human relatives - ArcticToday. ArcticToday. https://www.arctictoday.com/cold-tolerance-among-inuit-may-come-from-extinct-human-relatives/
- Huo, C., Song, Z., Yin, J., Zhu, Y., Miao, X., Qian, H., Wang, J., Ye, L., & Zhou, L. (2022). Effect of Acute Cold Exposure on Energy Metabolism and Activity of Brown Adipose Tissue in Humans: A Systematic Review and Meta-Analysis. Frontiers in physiology, 13, 917084. https://doi.org/10.3389/fphys.2022.917084
- Corliss, J. (2025, June). Cold plunges: Healthy or harmful for your heart? - Harvard Health. Harvard Health. https://www.health.harvard.edu/heart-health/cold-plunges-healthy-or-harmful-for-your-heart
- John-Paul Fuller-Jackson, Aimee L Dordevic, Iain J Clarke, Belinda A Henry, Effect of sex and sex steroids on brown adipose tissue heat production in humans, European Journal of Endocrinology, Volume 183, Issue 3, Sep 2020, Pages 343–355, https://doi.org/10.1530/EJE-20-0184
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
Winter Training
How The Fittest Do Winter
We analyzed 856,000 exercise sessions from Europeans across 20 countries to find out how people really train across the year. The data reveals three distinct exerciser phenotypes — and an entire category of athlete that didn't exist a decade ago, invented by indoor cycling platforms like Zwift.
April 9, 2026
