Sleep stages explained, and what Apple Watch shows
Sleep runs in cycles of about 80 to 100 minutes, usually four to six a night [1]. Each cycle passes through three stages of non-REM sleep, from light to deep, and then REM sleep, when most dreaming happens [1]. You get more deep sleep early in the night and more REM towards the morning [1].
Last reviewed 24 September 2026. General information from the Snorezy team, not medical advice.
The stages
Sleep medicine splits sleep into non-REM stages 1 to 3 and REM [1]:
- Stage 1 (N1): the transition from wake to sleep.
- Stage 2 (N2): you're asleep. Most of the night is spent here.
- Stage 3 (N3): deep, or slow-wave, sleep. More of it early in the night.
- REM: eyes twitch, the brain is active, and dreaming usually happens. Muscles normally go limp so you don't act out dreams. More of it later in the night.
What Apple Watch calls them
Apple Watch reports Awake, REM, Core and Deep. Core covers stages N1 and N2, what other trackers often call light sleep, and Deep is N3 [2]. Apple chose "Core" rather than "light" on purpose: N2 is normal, important, and often more than half the night, and "light" made it sound like a problem [2].
A watch estimates stages from sensors on your wrist. A sleep lab measures brain activity. The watch is useful for patterns over time; it isn't a lab result.
How much deep sleep is normal?
It varies a lot with age. Deep sleep peaks in early childhood, drops sharply in the teenage years and keeps decreasing through adulthood; some older adults have very little [1]. Less deep sleep than a younger person isn't, on its own, a sign that anything is wrong.
Waking between cycles
Brief awakenings between cycles are normal, and you usually don't remember them [1]. A tracker that shows a few awakenings a night is showing ordinary sleep. See why you wake up at night.
In Snorezy
Stages, next to the sounds
Snorezy reads your sleep stages from Apple Health and draws them under the night's sounds, so you can see what stage you were in when you snored, talked or woke. The stages come from Apple Health; Snorezy's microphone doesn't estimate them.
Questions
What is core sleep on Apple Watch?
Core is Apple's name for stages N1 and N2, the lighter stages of non-REM sleep. It's normal and usually the largest part of the night [2].
When does REM sleep happen?
Throughout the night, in each cycle, but there's more of it in the later cycles, towards morning [1].