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Sleep calculator

Tell it when your alarm goes off, or when you're going to bed. Get times that fit whole 90-minute sleep cycles, so you wake up between them.

To wake up at 7:00 AM between cycles, go to bed at one of these times:

  1. 9:45 PM6 cycles · 9h of sleep · recommended
  2. 11:15 PM5 cycles · 7h 30m of sleep · recommended
  3. 12:45 AM4 cycles · 6h of sleep
  4. 2:15 AM3 cycles · 4h 30m of sleep · a short night

Cycles average 90 minutes but vary from about 70 to 120, so treat these as a guide.

Bedtimes for common alarm times

Allowing 15 minutes to fall asleep. Five or six cycles is a full night for most adults.

Alarm6 cycles (9 h)5 cycles (7.5 h)4 cycles (6 h)
5:30 AM8:15 PM9:45 PM11:15 PM
6:00 AM8:45 PM10:15 PM11:45 PM
6:30 AM9:15 PM10:45 PM12:15 AM
7:00 AM9:45 PM11:15 PM12:45 AM
7:30 AM10:15 PM11:45 PM1:15 AM
8:00 AM10:45 PM12:15 AM1:45 AM

What happens in a sleep cycle

Each cycle moves from light sleep (stages N1 and N2) into deep, slow-wave sleep (N3), then into REM sleep, when most vivid dreaming happens, before surfacing briefly and starting again. Early in the night the cycles are heavy with deep sleep; toward morning, REM takes over. An alarm that lands in deep sleep leaves you groggy, a feeling called sleep inertia, which can last half an hour or more. One that lands as a cycle ends is easier.

Better nights, beyond the maths

  • Keep the same wake-up time every day, weekends included: it anchors your body clock more than bedtime does.
  • Watch the afternoon coffee. Caffeine has a half-life of about five hours; the caffeine calculator shows how much is left at bedtime.
  • Get daylight in the morning and keep the last hour before bed dim.
  • Short on sleep? A 20-minute power nap in the early afternoon helps without spoiling the night.

General guidance, not medical advice. If you're tired despite enough sleep, talk to a doctor.

Questions

How does the sleep calculator work?

Sleep runs in cycles of about 90 minutes, from light sleep into deep sleep and REM and back. Waking at the end of a cycle, in light sleep, feels easier than being pulled out of deep sleep. The calculator counts whole cycles back from your alarm (or forward from your bedtime), adding the time it takes you to fall asleep.

How much sleep do I need?

Most adults need at least seven hours a night, according to the American Academy of Sleep Medicine and the Sleep Research Society. That's five cycles (7.5 hours) for most people; six cycles (9 hours) suits some, and teenagers need more.

How long does it take to fall asleep?

Around 10 to 20 minutes for most people, so the calculator assumes 15. If you're asleep within a few minutes every night, you may be short of sleep; if it often takes more than half an hour, change the setting and consider what's keeping you up.

Is 90 minutes exact?

No. Cycles average about 90 minutes but range from roughly 70 to 120, and they change through the night: deep sleep dominates early cycles and REM the later ones. Treat the times as a good starting point and adjust by 15 minutes if you still wake groggy.

Further reading

  1. Sleep cycles explained: why 90 minutes mattersSleep runs in cycles of about 90 minutes through light, deep and REM sleep. What happens in each stage, why waking at a cycle's end helps, and how many you need.
  2. Caffeine half-life: when to have your last coffeeCaffeine's half-life is about five hours, so afternoon coffee is still working at bedtime. What's left of a cup over time, what changes it, and when to stop.
  3. Social jet lag: jet lag without leaving homeSocial jet lag is the weekly shift between work-day and free-day sleep. How to measure yours, why it matters, who gets it most, and how to reduce it.

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