Core
The lab's stages N1 and N2. N1 is the few minutes of drifting off; N2 is settled, light sleep, and where you spend most of the night. Apple chose "Core" over "light" because N2 is normal and important, not a lesser kind of sleep.
Sleep, explained
What are sleep stages?
The kinds of sleep your brain cycles through each night: light sleep, deep sleep and REM. Apple Watch calls them Core, Deep and REM. In 299 lab-scored nights from Apple's own study, about two thirds of sleep was Core, a fifth REM and an eighth Deep.
The sleep score is free · stages need an Apple Watch worn to bed
Four labels on your watch.
Sleep labs score every 30 seconds of the night. Apple Watch uses the same idea and the same names, with one difference: it folds the two lighter stages into one.
Deep
Stage N3, slow-wave sleep. Heartbeat and breathing reach their slowest and you're hardest to wake. It comes in longer stretches in the first half of the night.
REM
Rapid eye movement sleep, when most dreaming happens. The first REM comes about 90 minutes after you fall asleep, and the REM periods get longer towards morning.
Awake
Short wake-ups are part of a normal night, and most of them you never remember. The watch counts them so the night adds up.
Stage descriptions: NINDS, Brain Basics: Understanding Sleep. The labels and what each covers: Apple, Estimating Sleep Stages from Apple Watch.
How the watch knows
An estimate from your wrist.
A sleep lab reads brain waves. Apple Watch reads its accelerometer, sensitive enough to pick up the small movements of breathing, and classifies each 30 seconds.
Asleep or awake
In Apple's validation nights, the watch caught 97.9% of the time people were truly asleep, and 75% of the time they were truly awake.
Which stage
Mix-ups between very different stages were rare. The most common slip was deep sleep labelled as Core.
So, the trend
One night's deep sleep is a rough number. Weeks of nights, read against your own normal, say more.
Apple, Estimating Sleep Stages from Apple Watch (updated October 2025).
What Fullday shows.
Fullday reads the stages your Apple Watch saves to Apple Health, with nothing to sign in to, and turns the night into a sleep score from 0 to 100.
The night, drawn
A hypnogram of Awake, REM, Core and Deep, with the minutes in each, on the Sleep screen. Free
Stages in the score
Deep sleep and REM are two of the score's six parts, a tenth each. Deep is set against a typical share for your age, which falls as you get older. Free
Duration counts most
Hours slept against what your age needs weigh the most, then efficiency, restfulness and consistency. A short night holds the whole score down. How sleep is scored
No watch? Fullday still logs the night from Going to Bed and I'm Awake on your iPhone; those nights have no stages, so the score uses the parts it has. Want to know how many hours you need? Try the sleep calculator.
Questions about sleep stages.
What is Core sleep on Apple Watch?
Core is Apple's name for the lab's stages N1 and N2, what some trackers call light sleep. N2 often makes up more than half of a night, so lots of Core sleep is normal.
How much deep sleep should I get?
There's no single target. In Apple's lab-scored nights deep sleep averaged about 13% of sleep, with a wide spread, and its share falls with age (Ohayon et al. 2004). Compare your own weeks with each other rather than chasing a number.
Why is my REM sleep low some nights?
Most REM comes in the last hours of the night, so a short night or an early alarm cuts it first. Watches also estimate stages, so one night's REM is a rough number.
Do I need a watch to see my sleep stages?
For stages, yes: Fullday reads them from an Apple Watch worn to bed, or from another device whose app saves stages to Apple Health. An iPhone on its own logs when you slept, without stages.
Can sleep stages tell me if something is wrong?
No. Watch stages are a wellness estimate, not a sleep study, and Fullday never diagnoses anything. If you snore loudly, stop breathing in your sleep, or feel sleepy most days despite enough time in bed, talk to a doctor.
Next: what is a smart alarm? Or see how last night's sleep feeds Recovery.
Sources
Apple (updated October 2025). Estimating Sleep Stages from Apple Watch. apple.com/health
National Institute of Neurological Disorders and Stroke (last reviewed 2025). Brain Basics: Understanding Sleep. ninds.nih.gov
Ohayon MM, Carskadon MA, Guilleminault C, Vitiello MV (2004). Meta-analysis of quantitative sleep parameters from childhood to old age in healthy individuals. Sleep, 27(7), 1255–1273. doi.org/10.1093/sleep/27.7.1255
See your night, drawn.
Coming to the App Store this December. The sleep score and your stages are free with Apple Watch.
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