How they measure
Rings and watches both use an optical heart sensor and a motion sensor, and estimate sleep stages from heart rate, its variability, breathing and movement. Neither measures brain activity, which is what a sleep lab uses to define stages.
How accurate is it
Consumer devices are good at telling asleep from awake and at total sleep time, and less reliable at separating individual stages, deep sleep in particular, when compared with lab polysomnography. Treat stage minutes as estimates and trends as the useful part.
Choosing
- Ring: comfortable, many people forget they are wearing it, long battery life. No screen, no silent alarm on the wrist.
- Apple Watch: sleep stages since watchOS 9, a vibrating alarm that wakes you without waking anyone else, and it does everything else in the day. It needs charging, so the habit of charging it before bed or in the morning matters.
The best tracker is the one you wear every night: consistency beats a slightly better sensor worn half the time.
How Life OS uses your Watch
Life OS reads your sleep from Apple Health, merges every source into one night, scores it against your own normal, and graphs your heart rate, breathing and each part of the score through the night. When the Watch did not record a night, it reads the night from your heart rate instead.
Questions people ask
Is a ring or a watch better for sleep tracking?
Accuracy is similar because both use the same kind of sensors. Rings are more comfortable and last longer; a watch adds a silent wrist alarm and does more during the day. Wear whichever you will wear every night.
How accurate are sleep stages from a watch or ring?
Good for total sleep and for awake versus asleep, less reliable for individual stages such as deep sleep compared with a sleep lab. Trends are more useful than single nights.
Does Apple Watch track sleep stages?
Yes, since watchOS 9: awake, REM, core and deep, saved to Apple Health.