About REM sleep

What is REM sleep?

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

REM (Rapid Eye Movement) sleep is a stage of sleep where your eyes move quickly, dreams are vivid, and brain activity is high, almost like when you’re awake. During this time, your body is mostly paralyzed to stop you from acting out dreams. It’s important for memory, learning, and mood, and becomes longer in the later parts of the night.

Sleep stages was launched in WatchOS 9. Apple Watch Series 4 or later, including the SE, works with sleep stages.

Accuracy of Apple Watch’s REM sleep estimate

Apple developed their sleep stage algorithm using data from 858 volunteers, and then tested its accuracy on a separate set of 166 people who wore both an Apple Watch and medical PSG (polysomnography) equipment.

Apple Watch was about 78% accurate in detecting REM sleep. When it made a error, it was most likely to confuse REM sleep for core sleep (21% of the time). It rarely confused REM sleep for deep sleep and wake (<1% of the time each).

This is the full confusion matrix, showing the true stage (rows) vs what the Apple Watch predicted (columns):

How to measure REM sleep on your Apple Watch

All sleep stages-including REM—are automatically tracked as long as you sleep with your watch on for longer than 4 hours. You may need to set up a sleep schedule and turn on sleep tracking.

If my REM 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.

What is core sleep?

Core sleep is any sleep that’s neither deep nor REM. It’s also called “light sleep,” and corresponds to non-REM sleep stages 1 and 2. While REM sleep and deep sleep both have targets of how much to get, there’s no target for core sleep (i.e., it is not worth optimizing your core sleep—focus on REM and deep instead).