Sleep stages and the 90-minute cycle
A night of sleep is not one flat state. Learn the four stages, how they repeat in cycles, and why an early alarm costs you more dream sleep than you'd guess.
THE BASICS
Two kinds of sleep, four stages
Sleep comes in two kinds. Non-REM sleep has three stages, from light to deep: N1, N2 and N3. Then there is REM sleep (rapid eye movement), where your brain is almost as active as when you're awake and most vivid dreams happen. Each stage looks different on a brain recording and does a different job.
Dozing on the sofa and jerking awake when someone says your name is N1. Being so far under at 1am that the doorbell doesn't reach you is N3.
| Stage | What it's like |
|---|---|
| N1 | The doorway. A few minutes. Easy to wake, and you may insist you weren't asleep. |
| N2 | Light but real sleep. About half of the whole night. |
| N3 | Deep sleep. Slow, large brain waves. Very hard to wake, groggy if you are. |
| REM | Active brain, vivid dreams, body muscles switched off. |
Check yourself
Nima nods off in front of the TV. Five minutes later his sister turns the volume down and he says, 'Hey, I was watching that.' Which stage was he most likely in?
- REM, because he was probably dreaming about the show
- None: if he answered, he wasn't asleep at all
- N3, because he fell asleep so quickly
- N1, the light doorway stage people often don't notice they're in
Show the answer
N1, the light doorway stage people often don't notice they're in
Yes. N1 is so light that people woken from it often deny they were asleep. Five minutes in, he hadn't had time to go deeper.
THE PATTERN
The 90-minute cycle
You move through the stages in a loop: light sleep, down into deep sleep, back up, then a stretch of REM. One loop takes roughly 90 minutes, though anywhere from about 70 to 120 is normal. A full night is four or five cycles, often with a brief awakening between them that you don't remember.
Asleep at 11pm, up at 7am: about five cycles. Waking for a moment at 2am and again at 5am, rolling over and going straight back, is the seam between cycles, not a sleep problem.
The cycles are not identical
First half of the night
Dominated by deep sleep (N3). Your sleep pressure is at its highest, and deep sleep is what pays it off fastest. REM periods are short, sometimes only a few minutes.
Second half of the night
Dominated by REM and N2. Deep sleep has mostly been done. REM periods grow longer with each cycle, and the longest one comes just before your natural wake time.
Check yourself
Each 90-minute cycle contains the same mix of stages, so any 90 minutes of the night is worth the same as any other.
Show the answer
False
False. Early cycles are heavy on deep sleep and late cycles are heavy on REM. So the hours an early alarm removes are not average hours: they are the REM-rich ones.
An illustrative night, 11pm to 6:30am, five cycles.
REM minutes in each cycle:
cycle 1: 5
cycle 2: 15
cycle 3: 25
cycle 4: 30
cycle 5: 35
total: 110
Alarm moved from 6:30 to 5:00, so cycle 5 is gone:
sleep lost = 90 of 450 minutes
REM lost = 35 of 110 minutessleep lost: 20%
REM lost: about 32%The numbers are rounded and every night differs, but the shape is real: REM is back-loaded, so trimming the end of the night takes a bigger share of REM than of sleep.
Check yourself
Leila normally sleeps 11pm to 7am. For a week of early shifts she keeps the same bedtime but gets up at 5am. What is she mostly giving up?
- Deep sleep, because the body saves the deepest stage for the end of the night
- An equal slice of every stage, since cycles repeat
- REM sleep, which is concentrated in the last hours
- Only N1, the light stage that doesn't matter much
Show the answer
REM sleep, which is concentrated in the last hours
Right. Her deep sleep is mostly done by 3am. The two hours she cuts are where the long REM periods live.
Check yourself
Match each stage to its signature
Show the answer
- N1 → A few light minutes at the doorway of sleep
- N2 → Takes up about half the night
- N3 → Slow, large brain waves; hardest to wake from
- REM → Active brain, vivid dreams, muscles switched off
What this means for you
- Your brain takes its deep sleep first, whenever you fall asleep. So a short night is mostly paid for in REM and light sleep, and that loss is real.
- Brief awakenings between cycles are normal. They only become a problem if you can't get back to sleep.
- Being dragged out of deep sleep feels awful. That's one reason a long nap can leave you groggier than a short one.
- The only reliable way to get enough of every stage is to give sleep a long enough, regular window.
Check yourself
Deep sleep or REM?
- Most of it happens in the first half of the night
- Hit hardest by a 5am alarm
- Slow, large brain waves
- Vivid, story-like dreams
- Body muscles are switched off
- Pays off sleep pressure fastest
Show the answer
Deep sleep (N3): Most of it happens in the first half of the night, Slow, large brain waves, Pays off sleep pressure fastest
REM: Hit hardest by a 5am alarm, Vivid, story-like dreams, Body muscles are switched off
Lesson recap
- Sleep has four stages: N1 and N2 (light), N3 (deep) and REM (active brain, vivid dreams).
- They repeat in cycles of roughly 90 minutes, four or five times a night.
- Deep sleep dominates the first half of the night; REM dominates the second.
- Cutting the end of the night costs a bigger share of REM than of total sleep.
- You can't choose your stages. You protect them by giving sleep enough time.