N1 — falling asleep
A few minutes of transition. Muscles relax, thoughts drift, and a sudden "hypnic jerk" is common. You can be woken easily and may not realise you were asleep.
Sleep stages and how long a sleep cycle is
A sleep cycle lasts about 90 minutes and runs through three stages of non-REM sleep and one of REM. Here's what happens in each, and why the order matters for how you feel at the alarm.
+1 h · cycle 1 · Stage N3 (deep)
Slow-wave sleep, concentrated early in the night. Hardest stage to wake from — an alarm here causes the most grogginess (sleep inertia).
Drag across the chart. Simplified from the typical adult pattern (Carskadon & Dement; AASM scoring manual stage names).
The four stages
N2 — light sleep
The stage you spend most time in. Heart rate and body temperature drop, and the EEG shows sleep spindles — bursts linked to memory consolidation.
N3 — deep sleep
Slow-wave sleep. Hardest to wake from; growth hormone release peaks. Waking here brings the heaviest grogginess.
REM — dreaming sleep
Rapid eye movements, vivid dreams and near-waking brain activity, while most muscles are temporarily relaxed. Longest in the early-morning cycles.
Shares of a night are approximate for healthy young adults, following the figures summarised by Carskadon & Dement and the National Institute of Neurological Disorders and Stroke's "Brain Basics: Understanding Sleep".
How long is a sleep cycle?
The average adult cycle is about 90 minutes, and most fall somewhere between 80 and 110. The first cycle of the night is often a little shorter; later cycles run a little longer as REM stretches out.
An adult sleeping 7.5 hours usually completes four to six cycles. That's why the sleep cycle calculator works in 90-minute steps and lets you adjust the length.
Deep sleep comes first, REM comes last
Slow-wave (N3) sleep is concentrated in the first third of the night; by the fourth or fifth cycle there may be almost none. REM periods go the other way — the first might last ten minutes, the last can run half an hour or more. Cutting a night short costs you mostly REM; a late bedtime with a fixed alarm eats into both.
A single night already contains a small rhythm inside a bigger one. The sky has its own slow version: ahaboo's narrated walk-through of the Moon's phases shows how a month-long cycle comes from geometry rather than anything changing on the Moon.
How much deep sleep and REM do you need?
There's no official target for a single stage. In healthy adults, deep sleep is often about 13–23% of the night and REM about 20–25%, but both shift with age: deep sleep declines steadily from adolescence onward.
Consumer trackers estimate stages from movement and heart rate, and their stage-by-stage accuracy is limited compared with laboratory polysomnography. Use them for trends, not for grading individual nights.
The most useful thing you can do for both deep and REM sleep is simply to get enough total sleep, at regular times. Check your range on sleep needs by age.
Alcohol tends to suppress REM in the first half of the night and fragments the second half. Caffeine can reduce deep sleep even when you fall asleep fine — see the caffeine cutoff calculator.
Questions people ask
How long is a sleep cycle?
About 90 minutes on average for adults, typically between 80 and 110 minutes. Infants have shorter cycles of roughly 50–60 minutes.
How many sleep cycles do you need a night?
Most adults need four to six cycles — five cycles equals 7.5 hours, inside the 7–9 hour range recommended for adults.
What are the stages of sleep in order?
A typical cycle goes N1 → N2 → N3 → N2 → REM, then starts again. Early in the night N3 lasts longer; late in the night REM does.
Which sleep stage is best to wake up from?
Light sleep (N1 or N2) or right after REM, near the end of a cycle. Waking from deep N3 sleep produces the strongest sleep inertia.
Is deep sleep or REM more important?
Both do different jobs and neither can be skipped long term. Getting enough total sleep at regular times is the best way to get enough of each.