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Heat strain

BorrowedBased on research into a closely related measure, applied here to sauna and cold.
What it is
How hard your heart works at the same heat: your heart rate above resting, five minutes into your first heat round. It falls as you get used to heat.
How it is measured
The median heart rate from minute 4 to minute 6 of a session's first sauna or hot bath of 6 minutes or more, minus that day's resting heart rate from Apple Health. Later rounds start already warm, so only the first counts. Rounds are compared at the same heat: hot baths within 1 °C of measured water, saunas within 10 °C of the air temperature you typed in (or together, if you didn't). Your first 3 are set against your latest 3.
How to read it
Lower at the same heat suggests your body is handling heat better. In heat-acclimation research, heart rate under the same heat falls within 4–6 exposures, and nine 100 °C sauna sessions lowered heart rate in the sauna. Better fitness lowers it too, so read it next to resting heart rate.
What it cannot tell you
Most of the research is on exercise in the heat; the effect of passive heat alone is smaller and less certain. Sauna humidity, bench height, hydration, caffeine, a meal or a workout beforehand all move it. The water sensor is specified to 40 °C, so readings above that are less certain.

Sources

Every one of these ships inside the app, with the finding written out so you do not have to open the paper to know why it is there.

  1. Périard JD, Racinais S, Sawka MN. Adaptations and mechanisms of human heat acclimation. Scandinavian Journal of Medicine & Science in Sports, 2015.

    Repeated heat exposure lowers heart rate under the same heat stress; most of the change arrives in the first week.

    doi.org/10.1111/sms.12408

  2. Siquier-Coll J, Bartolomé I, Pérez-Quintero M, et al. Effects of exposure to high temperatures on cardiovascular parameters. International Journal of Environmental Research and Public Health, 2023.

    Over nine sessions in a 100 °C sauna, heart rate in the sauna was lower by sessions 8 and 9.

    doi.org/10.3390/ijerph20032082

  3. Kukkonen-Harjula K, Oja P, Laustiola K, et al. Haemodynamic and hormonal responses to heat exposure in a Finnish sauna bath. European Journal of Applied Physiology, 1989.

    At 80 °C, heart rate rose about 60% in dry air and about 130% once the air was made humid, which is why sauna rounds are only compared with each other.

    doi.org/10.1007/BF02330710

  4. Solomon S, Laye M. Post-exercise heat exposure and heat acclimation: a systematic review and meta-analysis. BMC Sports Science, Medicine and Rehabilitation, 2025.

    Heat after exercise lowered heart rate in later heat tests by a small amount, with low certainty.

    doi.org/10.1186/s13102-024-01038-6