Sleep Calculator
Find the best bedtime or wake-up time based on 90-minute sleep cycles.
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How it works
Sleep happens in cycles of roughly 90 minutes, and waking at the end of a cycle — rather than mid-cycle — leaves you feeling more refreshed. Enter the time you must wake up and the calculator counts back in 90-minute blocks, adding about 15 minutes to fall asleep, to suggest several ideal bedtimes (typically 5 to 6 cycles, or 7.5 to 9 hours). Or pick "sleep now" to see the best times to set an alarm. All times are computed locally in your browser.
bedtime = wakeTime − (n × 90 min) − 15 min fall-asleep
Common use cases
- Setting a bedtime that lets you wake up between sleep cycles for an early meeting.
- Deciding what time to set your alarm if you go to sleep right now.
- Planning a consistent sleep schedule around 5-6 full cycles a night.
Frequently asked questions
Why 90-minute cycles?
A full sleep cycle through light, deep, and REM sleep averages about 90 minutes. Waking at the end of a cycle, instead of during deep sleep, tends to feel easier and less groggy. Individual cycles vary from 80 to 110 minutes, so treat the times as a guide.
Why add 15 minutes?
Most people take around 10 to 20 minutes to actually fall asleep after getting into bed. The calculator adds about 15 minutes so the cycle math starts from when you are likely asleep, not when you lie down.
Is this medical advice?
No. It is a general guide based on average sleep-cycle length. Sleep needs and cycle length differ between people. For ongoing sleep problems, consult a healthcare professional.