Heat strain: the same sauna, five minutes in, month after month

Lower heart rate at the same heat is the clearest signal that a body has adapted to heat. Measuring it outside a lab means controlling four things at once, and most of this metric’s design is that control.

What is measured

The median heart rate from minute 4 to minute 6 of a session’s first sauna or hot bath of at least six minutes, minus that day’s resting heart rate from Apple Health.

Subtracting resting heart rate is what makes the number comparable across months. Resting heart rate drifts — with fitness, with the season, with sleep — and a raw in-sauna heart rate would carry all of that drift. The difference above resting isolates what the heat is adding.

The four controls

Only the first heat round of a session. Later rounds start already warm: core temperature elevated, skin already vasodilated, some fluid already lost. The first round is the only one beginning from a state that resembles other days.

At least six minutes. The measurement window is minutes four to six, so a five-minute round has nothing to measure.

Matched temperature. Hot baths are compared within 1 °C of measured water. Saunas are compared within 10 °C of the air temperature you typed in — or grouped together if you never typed one.

Same modality. A sauna is never compared with a bath. The two put very different loads on the heart at the same nominal temperature.

The tolerances differ by an order of magnitude for a reason. A bath’s water temperature is measured by the watch, so the app knows it to within the sensor’s accuracy. A sauna’s air temperature is a number you read off a dial and typed in, and sauna dials are notoriously approximate, so a tighter tolerance would be false precision.

This is also the argument for typing in the sauna temperature. Without it, every sauna you use gets pooled — the 70 °C one at the gym and the 95 °C one at the club — and the trend is partly a record of where you went.

The sauna half has a hardware limit

Heat strain reads heart rate from minute 4 to minute 6 of the round, so the watch has to be on your wrist in the heat. Apple advises against wearing any Apple Watch other than Ultra in a sauna, against wearing Ultra in a sauna above 55 °C (130 °F), and against wearing any model in a steam room.

That makes the hot-bath half of this metric the one most people can use. Baths run 38–42 °C, and their temperature is measured rather than typed, which is why they get the tighter 1 °C matching tolerance anyway. The sauna half works for an Ultra in a sauna kept below 55 °C, or an infrared cabin, and not for a traditional Finnish sauna at 80 °C.

Why humidity is the hidden variable

The 1989 Finnish study is the one that should make anyone cautious here. At 80 °C, heart rate rose about 60% in dry air and about 130% once the air was made humid.

Same room. Same reading on the dial. Roughly double the cardiovascular response.

Contrast cannot see humidity. Nobody’s watch can. So a trend that appears to show worsening heat strain may be a record of someone else throwing water on the stones more often this month. This is the largest uncontrolled variable in the metric and the app names it in the limits.

What the research supports

Heat acclimation is well studied and the timeline is consistent: repeated heat exposure lowers heart rate under the same heat stress, and most of the change arrives in the first week. Four to six exposures is the usual figure.

In a sauna specifically, a 2023 study ran participants through nine sessions at 100 °C and found in-sauna heart rate lower by sessions eight and nine.

The gap between that research and this metric is why it carries the borrowed tier rather than established. Most heat-acclimation work studies exercise in the heat, which is a considerably stronger stimulus than sitting still in it. A 2025 meta-analysis of post-exercise heat exposure found it lowered heart rate in later heat tests by a small amount, with low certainty.

So: the direction is well supported, the mechanism is understood, and the effect size for passive heat alone is smaller and less certain than the exercise literature would suggest.

Reading it alongside the heat adaptation estimate

The app carries two heat measures and they answer different questions.

Heat adaptation is a model. It counts your recent days of heat, adds a fraction of what is left to gain for each, and decays it when you stop. It predicts.

Heat strain is a measurement. It reads your heart rate.

The adaptation page checks one against the other, and the app’s own instruction when they disagree is unambiguous: if it says adapted but your heat strain is not easing, trust your heart rate. A model that contradicts a measurement is a model that is wrong about you.

What moves it that is not adaptation

The metric names them: sauna humidity, bench height, hydration, caffeine, a meal, or a workout beforehand.

Bench height is the one people underestimate. The temperature gradient in a Finnish sauna between the bottom and top bench can be 30 °C or more, and the dial is usually near the top. Sitting lower for a month will improve your heat strain without improving you.

Better fitness lowers it too, which is why the app suggests reading it next to your resting heart rate. If both are falling, the sensible reading is general cardiovascular adaptation rather than heat-specific adaptation.

One more, on the bath side: the water temperature sensor is specified to 40 °C. A 43 °C bath is above specification, so the 1 °C matching tolerance is tighter than the measurement supports up there. Treat very hot bath comparisons as looser than they look.