iPhone & Apple Watch · sauna, plunge & spa
Sauna, plunge, spa. Know what each one is actually doing.
Sauna, cold plunge, and now hot tubs — each logged and scored on its own algorithm, because one number can’t honestly describe all three. Every value labeled Estimated. Every claim linked to its source.
v1.6 “Spa Mode” live on the App Store · Requires iOS 18.6 or later
See the app
One app for all three — each stress kept separate, each labeled Estimated.
Sauna, steam, and infrared on the heat side. Cold plunges. And now hot tubs and hot baths. Logged as HealthKit workouts, scored honestly, and explained in plain language.
In a sauna, nothing goes in the hot room — you enter the temperature and the Watch contributes heart rate from your wrist. In water, Hormetic reads submersion and water temperature from Apple Watch Ultra’s built-in sensor; on every other model you enter the temperature yourself.
How the score works
Three stressors. Three separate scales.
Each session is scored from 0.00 to 1.00 and always shown as Estimated — but heat, cold, and spa run on their own independently versioned algorithms. A 40 °C soak is not a 90 °C room, and one number can’t honestly describe both.
Sauna, steam & infrared
Log the session, enter the temperature, and Hormetic estimates a heat-dose zone from your heart-rate response and duration. The dry-sauna estimate is the calibrated one; steam and infrared are shown at lower confidence, because humidity is assumed rather than sensed.
Cold plunges
Shallow-water immersion with an Estimated cold dose on its own scale — never mixed with heat — alongside cited safety guidance that appears before you plunge, not after. Heart rate is shown for context but never scored: water changes it in ways a wrist reading can’t fairly judge.
Hot tubs & hot baths
Step into the water and Apple Watch Ultra starts the session itself, reading water temperature from its built-in sensor; on other models you start it and enter the temperature. Lift an arm to the rim and the clock keeps running — the rest of you is still in the water — while the walk to the tub and the towel-off afterward are never counted.
The zones
Where your session sat — estimated, not measured.
Heat sessions are split into three zones by your heart-rate response. They’re an estimate of where a session landed in the useful range — not a reading of any molecular event. Cold and spa have their own zones on their own scales; the three are never merged.
Warming Up
An early, gentle rise. Your body is easing into the stress, not yet under real load.
Approaching
A meaningful response is building — the working part of most sessions.
Activation
A strong heat-stress response. Useful in doses; the point past which more isn’t better.
The science, honestly
Built on peer-reviewed research — and what it actually says.
Most of the strongest findings are associations from observational studies, not proof of cause. Hormetic shows you the whole picture, including where the evidence is thin.
Heart & circulation
In a long-running study of Finnish men, more frequent sauna use was associated with lower cardiovascular mortality — an association, not proof of cause. Small randomized trials add the clearest causal signal: sauna alongside exercise lowered blood pressure more than exercise alone.
Brain & memory
Higher sauna frequency has been associated with lower dementia and Alzheimer’s risk. The original signal came from a male cohort; a later study of both men and women pointed the same direction. Observational still — worth knowing, not a promise.
Consistency beats intensity
One hot session isn’t a switch you flip. In a meta-analysis of heat acclimation, exposure count tracked with increases in intracellular HSP70; peak temperature did not. That’s why Hormetic scores your habit over time, not a single hero session.
Cold, said plainly
Cold-water immersion reliably reduces how sore you feel — but taken right after lifting, it can blunt strength and muscle gains. And it carries real, immediate risk: the cold-shock response peaks in the first seconds. Reduced soreness isn’t the same as better adaptation.
Hot water — and why the soak score stops
A small randomized crossover trial in people with hypertension found that 20 minutes of immersion lowered blood pressure over the following 24 hours. The honest detail: thermoneutral water did it too. Some of the effect is the immersion itself, not the heat — so Hormetic’s soak estimate stops growing past 40 °C and past 20 minutes, because that is where the published evidence stops. More time is not more dose. A soak is not a treatment for high blood pressure, and Hormetic doesn’t track or manage it.
Where we hold back. Pooled across randomized trials, the chronic effect on most cardiometabolic markers isn’t significant — the honest counterweight to the observational headlines above. The name Hormetic nods to hormesis and the mTOR pathway studied in heat-stress research, but the app estimates where your session sat in the useful range; it doesn’t measure autophagy, growth signaling, or any molecular event. It is a tracking tool, not a medical device, and doesn’t diagnose, treat, or prevent anything.
Privacy by architecture
Nothing to leak, because nothing leaves.
Sessions are stored as HealthKit workouts on your device, protected by Apple’s standard iOS encryption. There is no Hormetic server to send your data to, by design. Optional CSV exports are yours to keep, and nothing is ever shared unless you choose to share it.
Using it safely
Nothing goes anywhere Apple says it shouldn’t.
Heat
Hormetic never asks you to put your iPhone or Apple Watch in a sauna. In a sauna, temperature is a value you enter and the Watch contributes heart rate only. Apple designs iPhone and most Apple Watch models to work best in 0–35 °C (32–95 °F) ambient air, and lists wearing a Watch in a sauna or steam room among things to avoid — a sauna runs far hotter, so leave your devices outside it. Heat exposure carries cardiovascular and dehydration risk — check with a physician first if you have a heart condition, are pregnant, or take medication that affects heat tolerance.
Cold
Cold-water immersion is sharper: the cold-shock response peaks in the first ~30 seconds. Enter gradually, keep your airway clear, don’t hyperventilate or breath-hold beforehand, and never plunge alone. If you have a heart condition, high blood pressure, or are pregnant, talk to a doctor first.
Spa
Hot water is a real cardiovascular load. Hormetic’s in-app guidance caps a soak at 40 °C (104 °F) — the same ceiling Apple publishes for water temperature during underwater activities with Apple Watch Ultra (0–40 °C / 32–104 °F). Stand up slowly, hydrate, skip alcohol, and don’t soak alone — get out at the first sign of dizziness, nausea, or headache.
On your devices: Apple says Apple Watch Series 2 and later can be used for shallow-water activities like swimming in a pool or ocean, which covers a cold plunge. A hot tub is warmer than the 0–35 °C ambient range Apple states for most Apple Watch models, so keep soaks short. Avoid soap and soapy water, which Apple warns can affect the water seals; rinse with warm fresh water after a treated pool or tub, then dry thoroughly. Apple notes water resistance isn’t permanent, can diminish over time, and can’t be rechecked or resealed.
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Make every session count — and know exactly what it counted.
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