Sleep isn't one steady state. Through the night you move through repeating cycles of light sleep, deep sleep and REM sleep, each lasting about 90 minutes on average, four to six times a night. Where you are in a cycle when the alarm goes off is a big part of whether you wake up refreshed or groggy.
The stages of sleep
| Stage | What happens | Share of the night (adults) |
|---|---|---|
| N1 | Light sleep: the drift from waking to sleep, a few minutes | About 5% |
| N2 | Lighter sleep with sleep spindles; heart rate and temperature fall | About half |
| N3 | Deep, slow-wave sleep: hard to wake from; most in the first half of the night | About a fifth |
| REM | Rapid eye movement: vivid dreams, most in the second half of the night | About a fifth to a quarter |
Early cycles are heavy with deep sleep, which matters for physical recovery and some kinds of memory. Later cycles contain more REM, linked to emotional processing and learning. Cut a night short and you lose mostly REM; that's one reason a short night leaves you foggy and short-tempered.
Why 90 minutes
Ninety minutes is an average. Cycles range from about 70 to 120 minutes, differ between people, and change through the night. The number is useful as a rule of thumb rather than a precise clock. Nathaniel Kleitman, who helped discover REM sleep, even proposed that a similar 90-minute rhythm continues through the day; see ultradian rhythms.
Waking at the right moment
Being woken from deep sleep produces sleep inertia: grogginess, slow thinking and a strong urge to go back to sleep, which can last half an hour or more. Waking from light sleep at the end of a cycle is much easier. So it can help to aim your alarm at the end of a cycle. For a 7 AM alarm, allowing 15 minutes to fall asleep, the cycle-friendly bedtimes are 9:45 PM, 11:15 PM, 12:45 AM, 2:15 AM.
The sleep calculator works these out for any alarm or bedtime. Because cycles vary, treat its times as a starting point and adjust by 15 minutes if you still wake groggy.
How many cycles do you need?
Most adults need at least seven hours of sleep a night, according to the American Academy of Sleep Medicine and the Sleep Research Society: five cycles is 7.5 hours. Teenagers need more. Hitting the end of a cycle is a nice bonus; getting enough cycles matters far more.
Naps and cycles
The same logic explains the best nap lengths. A 10 to 20-minute nap stays in light sleep and avoids grogginess; a 90-minute nap covers a whole cycle. In between, you risk waking from deep sleep. Our post on how long a power nap should be has the details, and the power nap timer handles the alarm.
How sleep changes with age
- Babies spend about half their sleep in REM, in shorter cycles of around 50 to 60 minutes.
- Teenagers need about eight to ten hours, and their body clocks shift later, which is why early school starts are so hard.
- Adults need at least seven hours. Deep sleep declines gradually through adulthood.
- Older adults tend to sleep more lightly, wake more often, and get sleepy and wake up earlier. Their total need doesn't fall as much as their sleep does.
What breaks up your cycles
- Alcohol helps you fall asleep but suppresses REM early on and fragments sleep in the second half of the night.
- Caffeine late in the day makes sleep lighter, even if you don't notice; see caffeine's half-life.
- Noise and light pull you up into lighter stages. A dark room and steady background sound help.
- An irregular schedule. Changing bed and wake times confuses the body clock that times your cycles; see social jet lag.
Rested, then focused
A good night is the best focus tool there is. When you're up, Flight turns your first focus session into a real flight: pick a destination and work until you land.