Sleep calculator
When to go to bed or wake up, in full 90-minute sleep cycles.
About this calculator
This sleep calculator suggests bedtimes and wake-up times that fit full 90-minute sleep cycles. Waking at the end of a cycle, rather than in the middle of one, can help you feel less groggy.
Tell it when you need to wake up, or when you're going to bed, and allow for the time it usually takes you to fall asleep.
How it works
Counts back from your wake-up time, or forward from your bedtime, in 90-minute sleep cycles, and allows for the minutes it takes you to fall asleep.
Last reviewed:
Worked example
- I want to: Wake up at a set time
- Wake-up time: 07:00
- Time to fall asleep: 15 minutes
Go to bed at: 11:15 PM (5 cycles, 7.5 hours of sleep)
Common questions
How much sleep do adults need?
Most adults need 7 or more hours a night, and many feel best with 7 to 9. Children and teenagers need more.
Why are sleep cycles 90 minutes?
During the night you move through light sleep, deep sleep and REM sleep in a repeating pattern. Each round takes about 90 minutes on average, though it varies from person to person and through the night.
What if I wake up tired after a full cycle?
Cycle lengths differ, so try moving your alarm 10 to 15 minutes either way. A regular bedtime, a dark room and less caffeine late in the day often help more than timing.
Sources and further reading
The story behind it
Where the idea came from: REM sleep was discovered in 1953 by Eugene Aserinsky and Nathaniel Kleitman at the University of Chicago.
Watching sleepers' eyes move rapidly under their lids, Aserinsky and Kleitman found a stage of sleep linked to dreaming. A few years later, William Dement and Kleitman showed that sleep runs in repeating cycles of roughly 90 minutes. Planning your night in whole cycles comes from that research.
Why you can trust this calculator
- It runs in your browser. Nothing you type into it is sent anywhere.
- The method is explained on this page, so you can check it yourself.
- Free to use, with no sign-up.
- Results are estimates, not medical advice.
- Spotted a mistake? Tell us and we'll look into it.
After 15 years as a mathematics educator, I built CalculStore to give students, teachers, professionals and anyone working with numbers simple, reliable tools.
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