Contrast for heat acclimation
Contrast tracks heat acclimation two ways and keeps them apart. Heat strain is a measurement: your heart rate five minutes into your first heat round, above your resting rate, compared only at the same temperature. Heat adaptation is an estimate from your recent days of heat, built on the timings in acclimation research. When the two disagree, the app tells you to trust the heart rate. Hot baths make the better measurement, because their temperature is read from the water.
The measurement: heat strain
Contrast takes the median heart rate from minute 4 to minute 6 of a session's first sauna or hot bath of at least six minutes, and subtracts that day's resting heart rate from Apple Health. Only the first heat round counts, because later rounds start already warm. Baths are compared within 1 °C of measured water; saunas within 10 °C of an air temperature you type in.
In acclimation research, heart rate under the same heat falls over the first four to six exposures, and most of the change comes in the first week. Lower heat strain at the same temperature is the shape to look for.
The estimate: heat adaptation
Each day with 30 minutes or more of sauna or hot bath counts in full, and adds up to a quarter of what is left to gain. Shorter days count in proportion; under five minutes does not count. After three days without heat, the estimate fades by 2.3% a day, the figure a meta-analysis of acclimation decay reported.
It carries Contrast's theoretical label. The parameters come from studies; the model built from them has not been tested. Passive heat is also a weaker stimulus than exercising in the heat, which most of that research studied, so the estimate will tend to run ahead of you.
Why baths beat the sauna here
Heat strain needs heart rate during the round, which needs the watch on your wrist. Apple advises against wearing any Apple Watch other than Ultra in a sauna, and against Ultra above 55 °C, which rules out a traditional sauna for every model.
A hot bath has neither problem. It runs 38 to 42 °C, its temperature is measured by the watch rather than typed, and it gets the tighter 1 °C matching. Humidity, which roughly doubled the heart rate response in one dry-versus-humid sauna study, does not vary in a bath at all.
Training days
Heat after training is a common pattern, and it muddies the measurement: a hard session raises heart rate on its own. Contrast's nights comparison splits days with a workout of 20 minutes or more into training plus a session against training alone, which is the closest the data gets to holding training constant. None of this is a training plan, and nothing in the app says how much heat to take.
Common questions
How long does heat acclimation take?
In acclimation research, about three quarters of the heart-rate change arrives within four to six days of exposure, and it fades by about 2.3% a day once the heat stops. Most of that research used exercise in the heat, which is a stronger stimulus than sitting in it.
Can I measure heat acclimation in a sauna with Apple Watch?
Only with an Apple Watch Ultra in a sauna kept below 55 °C, which is Apple's limit. For most people a hot bath is the practical way to measure it, since the watch can stay on and the water temperature is measured.