Hormetic

Hormetic tracks how your body responds to heat and cold exposure, using heart rate and the temperature involved, with honest math and full transparency.

WHAT YOU GET

- Heart-rate tracking during a session, recorded as a standard HealthKit workout
- Heat: three color-coded zones (Zone 1, Zone 2, Zone 3) and an estimated heat-dose score from your heart rate, the temperature you enter, and session duration
- Cold: cold-plunge tracking with three calm zones — Entering, Moderate, Significant — and an estimated cold dose from your time in the water and the water temperature
- Spa (new): hot tubs and hot baths, with their own zones and an estimated soak dose from your time immersed and the water temperature. A separate algorithm from heat — a 40 °C soak is not a 90 °C sauna, and one score cannot honestly describe both
- Contrast: follow a sauna or a soak with a plunge and Hormetic notes the pattern with honest, recovery-framed context — never a combined-benefit claim
- Per-session detail: peak heart rate, time in each zone, calories, algorithm version
- Adapt: your weekly consistency, with estimated heat, cold, and soak dose as separate trends
- Full session history

HONEST BY DESIGN

- Every value the algorithm derives is labeled "Estimated"
- Heat, cold, and spa use separate, independently versioned algorithms; each session records the version that produced its values
- In cold and spa, heart rate is shown for context but never scored — water changes it in ways a wrist reading can't fairly judge
- The 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
- Historical sessions are never silently recomputed
- Grounded in published research, with tappable source citations; water safety and its risks are covered prominently in the app

PRIVATE BY ARCHITECTURE

Your data never leaves your device. No account. No analytics. No telemetry. No third-party SDKs. We do not sell personal information.

IMPORTANT — YOUR DEVICES

Hormetic never asks you to put a device anywhere Apple's own guidelines say it should not go.

For heat, Hormetic does not require you to place your Apple Watch or iPhone in any hot room — the temperature is a value you enter manually, and Apple Watch simply contributes heart rate from your wrist.

Cold plunges, hot tubs, and hot baths are shallow-water immersion, which Apple Watch's water resistance supports. Hormetic's own in-app guidance caps a soak at 40 °C (104 °F), well under the 55 °C (130 °F) figure Apple publishes for Apple Watch Ultra in a hot environment. As Apple recommends, rinse your Watch with fresh water after a treated pool or tub. On Apple Watch Ultra, submersion and water temperature are read by the watch's built-in sensor; on other models you enter the water temperature. Keep your devices within Apple's published guidelines — water resistance is not permanent, and depth limits apply.

IMPORTANT — YOUR HEALTH

Cold-water immersion carries real risk. The cold-shock response can cause gasping and raise heart rate and blood pressure — enter gradually, never breath-hold beforehand, and don't plunge alone.

Hot water is a real cardiovascular load. Stand up slowly, hydrate, skip alcohol, and don't soak alone. Get out at the first sign of dizziness, nausea, or headache.

Hormetic is not a medical device and does not diagnose, treat, or prevent any condition. Consult a healthcare professional before starting heat or cold exposure, especially with a heart condition, high blood pressure, or pregnancy.

REQUIREMENTS

iPhone running iOS 18.6 or later, and Apple Watch running watchOS 11.6 or later, which records your sessions and heart rate. Apple Watch Ultra adds automatic submersion detection and water-temperature sensing for cold plunges and soaks.