Sleep Stages Explained: Deep vs Core vs REM

Every night you cycle through four sleep stages — awake, REM, core and deep — roughly every 90 minutes. Each stage has a different job, and the balance between them is what decides whether you wake up refreshed or foggy. Here's what your sleep chart is actually telling you.

The four stages on your chart

Deep sleep — the body's repair shop

Deep sleep (slow-wave sleep) is when physical restoration happens: tissue repair, immune activity, and growth-hormone release. It's concentrated in the first half of the night, and it's the stage that makes you feel physically rested. Most adults get roughly 10–20% of the night in deep sleep — often less than an hour, which surprises people.

REM sleep — the brain's processing time

REM (rapid eye movement) is when most vivid dreaming happens and when your brain consolidates memories and emotions. It dominates the second half of the night. A typical share is around 20–25%. Cut your night short and REM is what you lose first — one reason a consistent sleep schedule matters. Curious what those REM dreams mean? Try AI dream interpretation.

Core sleep — the foundation

"Core" is Apple's name for light (N1/N2) sleep — and it's not filler. It makes up about half of your night and acts as the scaffolding between deep and REM cycles. Seeing a lot of core sleep is completely normal.

Awake — brief and normal

Everyone wakes briefly between cycles, usually without remembering it. Short awake spikes on your chart are normal; long or frequent awake blocks drag down your sleep efficiency and are worth investigating.

How much deep sleep do I need?

There's no magic number, but if you consistently see under ~10% deep sleep and you wake up unrefreshed, look at the usual suspects: alcohol in the evening (a notorious deep-sleep killer), late caffeine, irregular bedtimes, and a warm bedroom. The fix is rarely one trick — it's trends. Improve one habit, then watch whether your deep-sleep trend moves over two weeks.

Reading your stages in Lunomia

Lunomia takes the sleep stages your Apple Watch records and turns them into a chart you can read in seconds: the whole night on one timeline, totals and percentages per stage, and week/month trends so you can see whether changes actually work. Each night also gets a sleep animal — a long deep-sleep night might make you a bear, while a fragmented one looks more like a raccoon.

1. Download Lunomia free and connect Apple Health.
2. Wear your Apple Watch to bed (setup guide).
3. Open the app in the morning: your hypnogram, stage percentages and sleep animal are ready.
4. Use the Trends tab to compare deep and REM sleep across weeks.

See your sleep stages clearly

Deep, core and REM visualized every morning — free, private, on-device.