Sleep Cycles: When to Go to Bed and When to Wake

What a sleep cycle is

Through the night the brain moves through light sleep, deep (slow-wave) sleep and REM sleep, the stage most closely tied to vivid dreaming, and then back again. One pass takes about 90 minutes on average, and a full night holds four to six of them. The cycles are not identical: deep sleep is concentrated in the first half of the night, while REM periods grow longer towards morning.

Being woken from deep sleep often brings sleep inertia, a groggy, slow feeling that can last from a few minutes to half an hour. Waking from light sleep or the end of a REM period is usually easier. That is the whole basis for timing an alarm to the cycles.

How much sleep adults need

Sleep medicine bodies recommend at least seven hours a night for adults, with seven to nine hours as the usual healthy range; teenagers need more, around eight to ten. In cycles, five or six (seven and a half to nine hours) fits that range, four cycles (six hours) falls short, and most people cannot make up a regular shortfall with longer sleep at weekends.

Needs vary between people, so the best guide is how you feel: waking without an alarm on free days, staying alert through the afternoon and not dozing off when sitting quietly are good signs that the amount is enough.

Working out a bedtime

Start from the time you must be up, count back in 90-minute cycles, and add the time it takes to fall asleep, typically 10 to 20 minutes. To wake at 07:00 with 14 minutes to fall asleep, five cycles (7.5 hours) means being in bed by about 23:16, six cycles (9 hours) by 21:46, and four cycles (6 hours) by 00:46.

The same arithmetic works the other way: if you go to bed now, the alarm times that end a cycle are about 1 hour 44 minutes, 3 hours 14 minutes and so on after you lie down, every 90 minutes plus the time to fall asleep. Falling asleep instantly every night is not a sign of good sleep; it usually means you are short of it.

Why the 90-minute rule is only approximate

Ninety minutes is a population average. Real cycles run from about 70 to 120 minutes, vary between people and between nights, and change across the night. Multiplied over five cycles, a 10-minute difference per cycle becomes nearly an hour, so a calculated alarm time is a starting point rather than a precise target.

If a calculated time keeps leaving you groggy, move the alarm 15 to 20 minutes earlier or later for a week and notice which feels better. Phone apps that claim to detect your sleep stage from movement are estimating too, and are less accurate than they suggest.

What affects sleep more than timing

A regular schedule matters more than hitting a cycle boundary. Going to bed and getting up at the same times, including at weekends, keeps the body clock steady. Bright light in the morning and dim light in the evening help; screens matter mainly because they keep you awake and engaged, not only because of their light.

Caffeine has a half-life of about five hours in most adults, though it varies widely: 100 mg in a coffee at 15:00 leaves roughly 50 mg at 20:00 and 25 mg at 01:00. Alcohol may help you fall asleep but breaks up sleep later in the night. Loud snoring with pauses in breathing, or daytime sleepiness despite enough hours in bed, can be signs of a sleep disorder such as sleep apnoea and are worth raising with a doctor.

• Aim for five or six cycles, roughly 7.5 to 9 hours
• Count back from your wake time and add 10 to 20 minutes to fall asleep
• Treat the calculated time as a starting point and adjust it
• Keep the same bed and wake times every day
• Stop caffeine by early afternoon
• See a doctor about loud snoring, breathing pauses or constant tiredness