Hot tubs & hot baths · iPhone & Apple Watch
How long should you stay in a hot tub?
No study has settled a best length for a hot tub soak, so Hormetic doesn’t set one. Its Estimated soak dose stops rising at 20 minutes, where the best single-soak evidence sits, and at 104 °F (40 °C), the recommended ceiling. Those are where the estimate stops counting, not targets. Get out at the first sign of dizziness, nausea, or a headache.
Requires 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.
A count-up clock, not a get-out alarm
How the hot tub timer works, from Start to summary
Hormetic is a hot tub app and soak tracker for iPhone and Apple Watch. Hot tubs, spas and hot baths all log as Spa.
Before: set the water
On Apple Watch Ultra there’s nothing to set: the start screen is always on Auto-detect. On other Apple Watch models, enter your thermostat setting or tap “I don’t have a gauge.” Above the recommended ceiling, the ceiling notice moves under Start in orange; the score doesn’t get bigger. On those models, Read thermostat photo on iPhone can also send the setting to the Watch for review.
Start: tap Start, then get in
Tap Start, then get in. The workout begins after a 3-second countdown. On Apple Watch Ultra, the soak clock opens at the first detected immersion. It scores the water temperature the Watch measures, and sets aside any temperature saved in a favorite, routine or thermostat photo. On other Apple Watch models, the clock runs from the end of the countdown, so time before you get in counts too.
During: arm on the rim
Heart rate shows “Recorded, not scored.” When time stops adding to the estimate, an on-screen line reads “Full time dose — the estimate stops climbing here.” It doesn’t buzz, and it isn’t a goal. On Apple Watch Ultra, an arm on the rim doesn’t stop the clock. After 2 minutes out of the water, the Watch asks “Still soaking?” If the soak ends while the Watch is out, by End or an unanswered prompt, it’s saved up to when the Watch left the water or your last “yes.”
After: the summary on iPhone
It shows an Estimated soak dose from 0.00 to 1.00, an Estimated zone, time immersed (itself an estimate), total duration, and the water badged Entered or Measured. The Adapt tab charts Estimated immersed minutes on their own, never added to heat or cold.
Spa zones
Did that soak even do anything? What the zone can tell you
A zone says how much soak Hormetic estimates you logged, not what it did to you.
Entering
Estimated soak dose under 0.40: just in, or a short soak. Water temperature alone never gets past here.
Moderate Soak
Estimated soak dose from 0.40 to under 0.75. A soak with no water temperature is capped at an Estimated 0.70, so it stops here.
Significant Soak
Estimated soak dose of 0.75 or more. It needs a known water temperature and time together: time alone, or water near body temperature, never gets here.
A higher zone isn’t a goal to chase. Heat and cold keep their own scales: see the sauna guide and the cold plunge guide.
How long
Why does the soak estimate stop at 20 minutes?
Almost all hot-water research ran 60 to 90 minutes a session, several times a week, for weeks. Hormetic still scores soaks that long, but doesn’t suggest them.
The best single-soak evidence is a small 2025 trial (below). Its 40-minute soak at 104 °F (40 °C) did no better than its 20-minute one. So the time credit is full at 20 minutes. That cutoff is our choice, not a measured plateau: past it, you add heat load, not Estimated dose.
How hot
How hot should a hot tub be?
US federal consumer-safety guidance puts the recommended ceiling for a hot tub at 104 °F (40 °C). It advises a 100 °F (37.8 °C) maximum in pregnancy, and warns that water above 102 °F (38.9 °C) can harm a baby in the first three months. If you’re pregnant, ask your doctor first. Hormetic doesn’t pick a temperature. Above 104 °F it adds no credit and shows an “Above the recommended ceiling” card instead.
Temperature credit starts at 98.6 °F (37 °C), body core temperature. Time counts for more than temperature, because the temperature scale spans only about 5 °F and time is the part you choose. Both are our choices.
A sauna is 149–194 °F (65–90 °C) air, and water moves heat into a body far faster than air. So a soak has its own math, never combined with a sauna session.
After a workout
Is a hot tub good after a workout?
We don’t know. The research behind Hormetic’s spa scoring doesn’t address soaking after exercise, so the app makes no recovery, soreness or performance claim.
The research, with its controls
What hot-water studies found, and what they didn’t
Hot-water studies only; no sauna study is cited for a soak.
Twenty minutes hot, and a lukewarm soak that did as well
In a small randomized crossover trial, 16 adults with high blood pressure, all on medication, soaked 20 minutes at 104 °F (40 °C). Their 24-hour systolic pressure averaged about 6 mmHg below a seated no-soak control, and a 40-minute soak at about 98 °F (37 °C), near body temperature, lowered it by a similar amount. So the honest reading is “a soak helped,” not “the heat did.” A soak is not a treatment for high blood pressure, and Hormetic doesn’t track or manage it.
Five minutes, and a reason to stand up slowly
In 12 healthy men, a 5-minute soak at about 106 °F (41 °C) blunted baroreflex sensitivity, the reflex that steadies blood pressure when you stand. The authors named dizziness, falls and fainting as the hazard. It’s a small, short-term finding in men only, and one likely reason people feel lightheaded getting out.
Weeks of long soaks, and a small null
In Brunt 2016, 10 young, sedentary adults soaked about 90 minutes at 105 °F (40.5 °C), 4–5 times a week for 8 weeks. Artery widening (flow-mediated dilation) rose from 5.6% to 10.9%, with no change in 10 who soaked near body temperature. That’s a multi-week result, never a single soak’s. In Lovell 2008, six young men soaked in hot water until their core temperature reached about 102 °F (39 °C). Top-up soaks then held it there, for 90 minutes in all. Heat-shock protein 70, a cellular stress protein, didn’t rise significantly in their immune cells. So Hormetic makes no heat-shock claim at any duration.
The limits: these studies are small (6 to 20 people), two of the four enrolled men only, and the 20-minute result comes from medicated adults with hypertension. Both stopping points, the weights, the zone cuts and the 0.70 cap are our choices, and soak depth isn’t modeled. Publishing what didn’t work is the only honest way to be believed about the rest.
Soaking safely
Hot tub safety, plainly
Hot water is a real cardiovascular load, and 104 °F (40 °C) is the recommended ceiling. Drink water before and after; you can sweat without noticing. Skip alcohol, don’t soak alone, and get out if you feel drowsy or at the first sign of dizziness, nausea, or a headache. Stand up slowly.
If you are pregnant, have a heart condition, unstable or medicated blood pressure, or take anything that causes drowsiness or affects circulation, ask your doctor before soaking. Hormetic estimates your soak dose; it does not make hot water safe.
For what Apple publishes about Apple Watch, see the FAQ below.
Hormetic is a wellness tracker. It is a tracking tool, not a medical device, and doesn’t diagnose, treat, or prevent anything.
Questions, answered plainly
Frequently asked
Does Hormetic work as a hot tub timer?
Yes, as a count-up soak clock and log, not a get-out alarm. The Watch shows time immersed (itself an estimate); the iPhone summary adds total duration. When time stops adding to the Estimated soak dose, an on-screen line says so; it doesn’t buzz, and it isn’t a goal. A 120-minute cap ends a forgotten session.
How long should you stay in a hot tub?
No study has settled a best length, so Hormetic doesn’t set one. Its Estimated soak dose stops rising at 20 minutes, where the best single-soak evidence sits; that’s where the estimate stops counting, not a time to aim for. Whatever the clock says, get out at the first sign of dizziness, nausea, or a headache.
Can you wear an Apple Watch in a hot tub?
Hormetic doesn’t tell you where your Watch goes; that’s Apple’s guidance to give. Apple says most Apple Watch models work best in ambient temperatures of 32–95 °F (0–35 °C), and a hot tub is warmer than that. Apple also warns that soap and soapy water can affect the water seals, and that water resistance isn’t permanent and can’t be rechecked.
Does Apple Watch measure a hot tub’s water temperature?
Apple’s Depth app shows water temperature on Apple Watch Ultra and Series 10 and later. In Hormetic 1.7, automatic water-temperature reading is supported on Apple Watch Ultra; on other models you enter the temperature. With no gauge, the Estimated soak dose uses time alone, capped at 0.70. More in our explainer.
Why isn’t heart rate part of the Estimated soak dose?
A short hot soak has been shown to blunt the reflex that steadies blood pressure when you stand, so heart rate isn’t a clean read on heat load. Hormetic also hasn’t validated wrist readings in water. So heart rate is “Recorded, not scored,” and the Estimated soak dose uses only time immersed and water temperature.
Will my soaks show up in Apple Fitness?
Yes. Since Hormetic 1.7, new soaks, hot baths included, are titled “Hot Tub” in the Apple Fitness workout list instead of “Other.” The workout type is still Other, so ring behavior is unchanged. Older soaks keep “Other,” because HealthKit can’t change a saved workout’s name.
Keep reading
Each one on its own scale.
$2.99 · one-time
Track your next soak, on its own books.
$2.99 one-time: no subscription, no in-app purchases, no account. Requires 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. A wellness tracker, not a medical device.