Sleep cycle calculator
Bedtimes and wake times aligned to 90-minute sleep cycles.
Cycle length varies; treat these as approximate.
15 minutes is average
90 is typical; 80–110 is normal
Go to bed at one of these times
How it works
Sleep runs in repeating cycles through light sleep, deep sleep and REM. Waking at the end of a cycle — in light sleep — generally feels better than being pulled out of deep sleep, which produces the groggy, disoriented state called sleep inertia.
bedtime = wake time − (cycles × cycle length) − time to fall asleep Why 90 minutes is a rough average
The 90-minute figure comes from early sleep-lab work and is a population mean. Individuals range from about 70 to 120 minutes, and cycles lengthen as the night progresses — early cycles are deep-sleep heavy, later ones REM-heavy. That is why cutting the last two hours of sleep costs disproportionately much REM.
Total sleep beats cycle timing
Six cycles at 90 minutes is nine hours; five is seven and a half. Most adults need seven to nine hours, so the useful advice is usually “aim for five or six cycles” rather than “engineer the exact minute”. Four cycles is six hours — survivable, not sustainable.
What actually helps
A consistent wake time, seven days a week, does more for sleep quality than any bedtime calculation. Light in the morning, dark and cool in the evening, caffeine cut-off in the early afternoon, and no alcohol close to bed — those are the levers with real evidence behind them.