Cold Plunging Might Make Your Biomarkers Worse

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)

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

1. 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/
2. 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
3. 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
4. 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

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