CircadiaOSShort answer: Apple Watch Series 8 and later, Apple Watch SE 3, and all Apple Watch Ultra models measure wrist temperature while you sleep and show how far each night is from your personal baseline. You need to wear the watch to bed with Sleep Focus on, and it takes about five nights to set the baseline. It is not a fever thermometer, and a warmer room or heavier bedding can move it.
How wrist temperature works
The watch has temperature sensors on the back, against your skin, and on the front under the display. Apple uses both to reduce the effect of the outside environment. It samples your temperature overnight and, after about five nights, reports each night as a change from your baseline in the Health app (under Body Measurements) and in the Vitals app on newer watchOS versions.
Like Oura's reading, this is skin temperature, not core temperature. It is good at showing that tonight was different from your normal. It is not built to tell you your body temperature.
Which models have it
- Yes: Apple Watch Series 8 and later, Apple Watch SE 3, Apple Watch Ultra (all models).
- No: earlier Apple Watch SE models and Series 7 or earlier. These still track sleep stages on watchOS 9 or later, just not temperature.
What moves the number
- The room and bedding. Skin temperature responds to how easily heat can leave your body. A warmer bedroom, a heavy duvet or the watch arm tucked under a pillow can all raise it.
- Alcohol, a late meal or a late workout. All raise skin temperature for a few hours.
- The menstrual cycle. Apple uses wrist temperature for retrospective ovulation estimates in Cycle Tracking, because temperature rises after ovulation.
- Getting sick. A rise across several nights can show up here, alongside other changes in the Vitals app.
Using it to check your bedroom
- Open your thermostat app's history and write down the bedroom temperature for the last two weeks of nights.
- In the Health app, look at wrist temperature for the same nights.
- If the higher readings line up with warmer nights, and nothing else (alcohol, illness, cycle) explains them, the room is probably part of it.
- To test it, change one thing, such as a 2°F cooler first half of the night, and hold it for a week or more. Single nights are too noisy.
Can Apple Watch control my thermostat?
Not directly for sleep. You can use a Sleep Focus or Apple Home automation to set a HomeKit thermostat to one temperature at bedtime, but that holds one number all night. It does not cool further for deep sleep, warm before your alarm, or change based on how you slept.
Where CircadiaOS fits
CircadiaOS is an iPhone app that reads your Apple Watch sleep data after each night and sends a four-phase overnight schedule to your smart thermostat (Nest, ecobee, Honeywell Home and 20+ more brands) each evening. On Premium, after a 14-night starting period, it changes one setpoint at a time by about 2°F, holds it for about 10 nights (the Apple Watch block length), and keeps the change only if your sleep data over that stretch supports it. It works between nights, not in real time, never goes below 63°F, and does not use your temperature data to diagnose anything. Apple Watch setup guide.
Questions
Does Apple Watch measure body temperature?
It measures wrist skin temperature while you sleep, on Series 8 and later, Apple Watch SE 3 and Apple Watch Ultra, and shows it as a change from your baseline. It does not give you a core body temperature and is not meant for checking a fever.
Why is my Apple Watch wrist temperature high?
Common reasons are a warmer bedroom, heavier bedding, alcohol, a late meal or workout, the second half of the menstrual cycle, or getting sick. If it stays high for several nights and you feel unwell, talk to a doctor.
Does room temperature affect Apple Watch wrist temperature?
It can. Apple uses two sensors to reduce the effect of the environment, but skin temperature still depends on how easily heat leaves your body, which the room and your bedding affect.
How long does Apple Watch take to show wrist temperature?
About five nights of wearing it to sleep with Sleep Focus on. After that, each night appears as a change from your baseline in the Health app.
Can my Apple Watch change my thermostat at night?
Only in a basic way: an automation can set one temperature at bedtime. For a temperature that changes through the night and adjusts from your sleep data, you need something like CircadiaOS connected to a smart thermostat.
Sources
- Apple Support, wrist temperature on Apple Watch (support.apple.com).
- Apple Newsroom, Apple Watch Series 8 announcement, September 2022 (apple.com/newsroom).
- Harding EC, Franks NP, Wisden W. The temperature dependence of sleep. Frontiers in Neuroscience, 2019.
Related guides
- Connect Apple Watch to your thermostat — the setup.
- Oura temperature and your thermostat — the same question on Oura.
- Why do I wake up hot? — common causes.
- Best temperature for sleep — the research.