How it works
Sleep runs in cycles of light, deep and REM sleep that last roughly 90 minutes on average. Waking at the end of a cycle, rather than in the middle of deep sleep, may help you feel less groggy.
This calculator counts back (or forward) in 90-minute blocks and adds the time it usually takes you to fall asleep. Five or six cycles gives 7.5 to 9 hours, in line with the 7 to 9 hours recommended for adults by the American Academy of Sleep Medicine.
Real cycles vary from about 70 to 120 minutes and change through the night, so treat the times as a guide. Results are estimates, not medical advice.
Formula
Bedtime = wake time − (cycles × 90 min) − time to fall asleep
Questions people ask
How long is a sleep cycle?
About 90 minutes on average, though individual cycles range from roughly 70 to 120 minutes. Early cycles have more deep sleep; later ones have more REM sleep.
How many sleep cycles do I need?
Most adults do well with five or six cycles, which is 7.5 to 9 hours. Fewer than four cycles (6 hours) on a regular basis falls short of common recommendations.
How long should it take to fall asleep?
Typically 10 to 20 minutes. Falling asleep in under five minutes can be a sign of sleep debt.