It turns out that the new wrist temperature sensing capability, which is available on Apple Watch Series 8 and Apple Watch Ultra models, requires users to wear their watches for five nights before they can detect a baseline temperature, according to a new support document issued by Apple today.
The temperature sensors of the Apple Watch Series 8 and Apple Watch Ultra are located right below the display and on the rear crystal, respectively, close to your skin. Every five seconds, the Apple Watch measures the wearer’s wrist temperature while they are sleeping. According to Apple, this architecture reduces bias from the surrounding environment, improving accuracy.
Your body temperature normally changes throughout the day and might change at night depending on your lifestyle choices, including what you eat and how much you exercise, how you drink, where you sleep, and physiological factors like menstrual cycles and illness. Your Apple Watch will establish your wrist temperature baseline after around 5 nights and check for nightly variations to it.
According to Apple, in order for Sleep to function properly, both Sleep Focus and “Track Sleep with Apple Watch” must be turned on for at least four hours per night for roughly five nights. After that, users can check for recorded measurements in the Health app’s Body Measurements-> Wrist Temperature section.
Apple issues a warning that the function is not a thermometer, is not a medical-grade gadget, and cannot offer measures when requested. It further states that it should not be used for any medical purposes. Data on wrist temperature can potentially be affected by an Apple Watch that fits loosely.
The feature is positioned by Apple as a way to enhance period predictions and retrospective ovulation estimates in marketing materials for the new Apple Watch models, but the supporting documentation implies that monitoring nightly temperature changes while sleeping can provide insight into anyone’s “overall well-being.”
Wrist temperature may be disabled in the Watch app on an iPhone under Privacy -> Turn off Wrist Temperature if you don’t need it.