About deep sleep

Deep sleep is the middle stage of each sleep cycle. During this stage, your body builds bone and muscle, repairs tissues, and strengthens the immune system. Low deep sleep may be caused by bad sleep hygiene or by health conditions such as sleep apnea.

If deep sleep repairs the body, REM sleep repairs the mind.

How to track deep sleep on Apple Watch

Any Apple Watch on watchOS 9 or above can track sleep stages. Deep sleep is automatically tracked as long as you sleep with your watch on for more than 4 hours, and sleep focus mode is enabled. The easiest way is to turn on sleep focus each night is to set a sleep schedule. If you followed these steps don’t see sleep stages, you may need to turn on sleep tracking in settings.

Sleep stages was introduced in WatchOS 9. Any Apple Watch Series 4 or later, including the SE, is compatible with sleep stages.

Accuracy of Apple Watch’s deep sleep estimate

The Apple Watch’s sleep stage algorithm was tested in a study of 166 people who wore both an Apple Watch and polysomnography equipment.

Apple Watch was about 62% accurate in detecting REM sleep, confusing it for core sleep 38% of the time. This means your Apple Watch is likely to underestimate deep sleep. The numbers above giving normal values are based on Apple Watch data, so they take into account this likely understimate.

This is the confusion matrix from the study. Rows show the true stage (rows) and columns show what sleep stage the Apple Watch predicted:

If my deep sleep is low, can that be a sign of sleep apnea?

Potentially. You can talk with an Empirical Health doctor about sleep apnea testing by booking an appointment.