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Sleep cycle calculator

Work out bedtimes or wake-up times that line up with full 90-minute sleep cycles.

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Sleep cycle calculator
Enter a wake-up time or a bedtime and this calculator works backward or forward through full sleep cycles to suggest when to fall asleep or when to wake up. Waking up mid-cycle, during deep sleep, is a common reason an alarm can leave you feeling groggier than one set just a few minutes earlier or later — spacing an alarm across whole cycles is meant to land it closer to a lighter stage of sleep instead.
Switch "Calculate from" between "I want to wake up at" and "I'm going to sleep at" depending on which time you already know. Cycle length defaults to the commonly cited 90 minutes but can be adjusted, and the time-to-fall-asleep option accounts for the minutes it typically takes to actually drop off before the first cycle starts. Minimum and maximum cycles widen or narrow the list of suggested times, the time format switches between 24-hour and 12-hour notation, and nap mode swaps the cycle range for two fixed nap lengths — a short 20-minute power nap and a full 90-minute cycle nap.
Results that land before midnight or past it are marked as the previous or next day, so a bedtime suggested for a very early wake-up time is never mistaken for the same evening. Every suggested time also lists how many cycles it covers and the total time asleep, and the output can be plain text or a simple table for pasting into a planner or spreadsheet.
Everything runs in your browser — no time you enter is uploaded or stored anywhere. Copy the result, download it as a .txt file, or send the output back into the input to try a different scenario.
FAQ
Why 90 minutes for a sleep cycle?
90 minutes is the commonly cited average length of one full sleep cycle through the light, deep and REM stages, though it varies a little from person to person. Adjust the cycle length option if you know yours runs shorter or longer.
Why does it add extra minutes before counting cycles?
Falling asleep isn't instant. The "time to fall asleep" option adds that buffer before the first cycle begins, so a wake-up time doesn't assume you were already asleep the moment you got into bed.
Should I aim for 4, 5 or 6 cycles?
More completed cycles generally means more total sleep, but the point of the calculator is to avoid waking mid-cycle — pick whichever count in the list fits the time you have, rather than always choosing the highest number.
What does nap mode change?
Nap mode replaces the customizable cycle range with two fixed options: a 20-minute power nap that avoids deep sleep, and a 90-minute nap that completes one full cycle.
Is my input uploaded anywhere?
No. The calculation runs entirely in your browser — the time you enter never leaves your device.