Apple Watch Now Tracks Sleep Phases Too: What Do They Mean?

The new watchOS 9 has made your Apple Watch a better sleep tracker. You can now also track your sleep phases: deep sleep, core sleep and REM sleep. But what does it mean?

Sleep Phases in watchOS 9

If you have an Apple Watch on your wrist at night, the Sleep app accurately keeps track of the different phases. This way you not only know exactly when you slept, but also what the quality of your night’s sleep was.

To measure your sleep, open the Sleep app on the Apple Watch. There, follow the on-screen steps to enable the feature. You can now see your sleep statistics in the Health app. Make sure you have watchOS 9 installed.

Here’s how to view your sleep stats:

What are Sleep Phases?

Every night you go through several sleep cycles, also called stages. According to experts, there are four: awake, light sleep, deep sleep and REM. The Apple Watch also measures these sleep phases, although Apple chooses to use the term ‘Core sleep’ instead of ‘Light sleep’.

Below we explain what the four sleep stages mean.

1. Core Sleep

The sleep cycle begins with core sleep, also known as light sleep. You can be easily awakened at this stage. This sleep phase often lasts a little less than an hour. It is beneficial to wake up in this phase. If you wake up naturally during light sleep, you often feel extra rested.

2. Deep Sleep

After your light sleep you move on to the most important phase: the deep sleep. It is more difficult to wake someone up at this stage of the sleep cycle. It is an essential phase as your body builds and repairs muscle, bone and tissue. Plus, it helps keep your immune system functioning.

In adults, deep sleep makes up about 25 percent of the night’s rest. Deep sleep lasts about 20 minutes per cycle.

3. REM sleep

REM sleep is also called dream sleep, in which there are rapid eye movements and a large brain activity. The muscles are completely relaxed. The brain is active with dreams and processing information. This sleep phase is considered less important.

An adult is in REM sleep for 15 to 25 percent of their sleep.

New cycle

After REM sleep you sometimes wake up briefly and the sleep cycle starts again from the beginning. To measure sleep phases, the smartwatch uses data from the accelerometer and heart rate monitor.