Sleep Calculator
Enter the time you must wake up or plan to go to bed — get times that land at the end of a 90-minute sleep cycle.
Reviewed by the OmniCalc teamMethod verified 2026-07-01
21:45bedtime
Best bedtime 21:45 for 6 sleep cycles (9 hours)- 5 cycles · 7.5 h
- 23:15
- 4 cycles · 6 h
- 00:45
- 3 cycles · 4.5 h
- 02:15
- 6 cycles · 9 h
- 21:45
Show steps
- To wake refreshed at 07:00, count back 15 min to fall asleep plus whole 90-minute sleep cycles.
- 6 cycles · 9 h of sleep → go to bed at 21:45.
- 5 cycles · 7.5 h of sleep → go to bed at 23:15.
- 4 cycles · 6 h of sleep → go to bed at 00:45.
- 3 cycles · 4.5 h of sleep → go to bed at 02:15.
Based on ~90-minute sleep cycles and 15 minutes to fall asleep — a general guide, not medical advice. Real cycle length varies from person to person.
How to use the sleep calculator
- 1Choose whether you’re planning around a wake-up time or a bedtime.
- 2Pick the clock time in the time field.
- 3Read the recommended time (6 cycles · 9 hours) plus shorter-night alternatives, with the working under Show steps.
Wake between cycles, not mid-cycle
Waking in the middle of a cycle — especially out of deep sleep — is what makes an alarm feel brutal. Aim for one of these times so the alarm lands in the light phase between cycles; a shorter night ending on a full cycle often beats a longer one cut off mid-cycle.
Frequently asked questions
How does a sleep cycle calculator work?
Sleep moves through roughly 90-minute cycles, and waking at the end of a cycle feels better than waking mid-cycle. This tool adds 15 minutes to fall asleep, then counts whole 90-minute cycles to suggest times at 9, 7.5, 6 and 4.5 hours of sleep.
How many hours of sleep should I get?
Most adults do best on 7 to 9 hours — that is 5 to 6 full cycles (7.5 h or 9 h) on this calculator. The 4.5-hour option is a last resort for a short night, not a target to aim for.
Why 90 minutes and 15 minutes to fall asleep?
A full cycle (light → deep → REM sleep) averages about 90 minutes, and people take roughly 15 minutes to drift off. Both are averages, so your own cycle may run a little shorter or longer on any given night.