Apple Watch Ultra and Apple Watch Series 8 feature a new sensor that measures both skin and environment temperature. Apple has said (and wrote in its patent) that detecting skin temperature reliably actually requires two sensors, one on the back crystal in contact with your skin and the other just under the display, to pick up environment temperature. The algorithm is fed data from the two sensors to accurately calculate the temperature on the surface of your skin.
Apple’s support document clarifies that a five-day calibration period is required before the watch can establish your baseline wrist temperature.
“After about five nights, your Apple Watch will determine your baseline wrist temperature and look for nightly changes to it,” it reads.
How to track wrist temperature changes with Apple Watch
Tracking wrist temperature and related features require Apple Watch Series 8 or Apple Watch Ultra. Older models don’t support this functionality. You’ll need to set up the Sleep feature with the “Track Sleep with Apple Watch” option turned on. Be sure to also turn on Sleep Focus for at least four hours per night for about five nights.
You can view saved measurements by launching the Health app on your iPhone, then go to Browse → Body Measurements → Wrist Temperature.
If the sensor isn’t calibrated, you’ll see “Needs More Data” at the top of the chart instead of actual readings. You will also spot the number of nights remaining before your wrist temperature data is available right below the chart.
How to interpret saved wrist temperature data
An aggregate for each night is displayed in the Health app as relative changes from your established baseline wrist temperature. Because fluctuations in body temperature each night can be indicative of illness or menstrual cycles, relative changes give you better insight into your overall well-being.
Apple Watch Series 8 and Ultra also sport cycle tracking which predicts when you’ve ovulated by monitoring changes in your wrist temperature while sleeping.
When you’ll need to re-calibrate the sensor
This five-night calibration only needs to be done once per watch. Once the sensor has been calibrated, the watch will take wrist temperature readings every five seconds. Apple explains that this improves accuracy by reducing bias from the outside environment.
If the watch is repaired or you pair a new one, you’ll need to re-calibrate the sensor again over five nights. Things that can throw the sensor off include loose fit (be sure the back crystal is in contact with your wrist), your age (you must be at least 14 years old) and “certain physiological, lifestyle and environmental factors.”
How to turn off wrist temperature and tracking
To turn off wrist temperature, open the Watch app on your iPhone and go to My Watch → Privacy → Wrist Temperature, then toggle off the feature from there.
To disable nighly wrist temperature tracking, open the Health app on iPhone and go to Browse → Cycle Tracking → Options, then turn off “Use Wrist Temperature.” Doing so will also disable retrospective ovulation estimates and period predictions.
Can I take wrist temperature readings on-demand?
No, you cannot use your Apple Watch to measure wrist temperature anytime you want like you can your heart rhythm. The device automatically polls the sensor every five seconds when Sleep Focus is active. You get to see a history of relative changes to your established wrist temperature in the Health app, but that’s about it.
Does Apple Watch double as a thermometer?
Nope. Neither Apple’s temperature sensor nor its algorithm act as a thermometer.
Is Apple Watch a medical device?
Just because it can measure your heart rate or your wrist temperature doesn’t mean Apple Watch is a medical device. Apple clearly spells out in black and white that the temperature-sensing feature “isn’t intended for use in medical diagnosis, treatment or for any other medical purpose.”