Sleep cycle calculator
Bedtimes that end on a full sleep cycle.
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How it's calculated
Sleep runs in cycles of roughly 90 minutes, light to deep to REM and back, and waking at the end of a cycle feels far better than being pulled out of deep sleep in the middle of one. Counting back from the wake-up time in 90-minute steps, plus the time it takes to fall asleep, gives bedtimes that land between cycles: six cycles is nine hours of sleep, five is seven and a half, four is six. Switch to a known bedtime and the same steps run forward to the wake-up times. Times are shown on a 24-hour clock as an hour and a minute.
How many sleep cycles do I need?
Five to six for most adults, which is 7.5 to 9 hours, in line with the 7–9 hours health bodies recommend. Four is a short night you can get away with occasionally; three is only for a nap-and-go emergency. Teenagers need six; older adults often do well on five.
Is every cycle exactly 90 minutes?
No. Cycles range from about 70 to 120 minutes and tend to lengthen through the night, with more deep sleep early and more REM later. Treat the times here as a 15-minute window and notice when you wake up refreshed; that is your own cycle length showing.
Why do I feel worse after more sleep?
Usually because an alarm caught you in deep sleep, which causes sleep inertia: grogginess that can last 30 minutes or more. Waking at the end of a cycle, even after less total sleep, avoids it. A consistent wake time every day, weekends included, does more for alertness than any single night.
Results are estimates for informational purposes only and are not financial, medical or legal advice.