Have you ever slept a full eight hours and still woken up groggy, while another day you felt fine after six? Timing can be part of the reason. Sleep runs in cycles of roughly 90 minutes, moving through light, deep and REM sleep, and waking at the end of a cycle usually feels easier than being pulled out of deep sleep halfway through one. This free sleep calculator uses that idea in two ways. Tell it the time you need to wake up and it suggests bedtimes that give you 4, 5 or 6 complete cycles. Or tap "I'm going to bed now" and it shows the best times to set your alarm. It adds an average of 15 minutes to fall asleep, and you can change that if you usually drift off faster or slower. Six cycles (about 9 hours in bed) or five (about 7.5 hours) are highlighted, because most adults need 7 to 9 hours of sleep. Treat the results as a helpful guide rather than a precise science: cycle length varies from person to person and through the night. If you're regularly tired despite enough sleep, talk to a doctor. Nothing you enter leaves your browser.
Plan your sleep
Go to bed at one of these times:
Highlighted: 5–6 cycles (about 7.5–9 hours), the range most adults need.
How the sleep calculator works
- Each sleep cycle is taken as 90 minutes.
- Your time to fall asleep (default 15 minutes) is added.
- Times are counted back from your alarm, or forward from now, in whole cycles.
Better sleep basics
- Keep the same wake-up time every day, weekends included.
- Avoid caffeine late in the day and bright screens before bed.
- Keep your bedroom cool, dark and quiet.
FAQ
Why 90 minutes?
It’s the commonly cited average length of a sleep cycle. Real cycles vary from about 70 to 120 minutes.
How much sleep do I need?
Most adults need 7–9 hours; teenagers and children need more.
Is waking between cycles proven to help?
It’s a popular and reasonable guide, but evidence is limited. Consistent, sufficient sleep matters most.
Is anything stored?
No. It runs in your browser.
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