Sleep Calculator: Bedtimes That Complete Full Sleep Cycles
Waking groggy often means an alarm interrupted a deep cycle. Counting backwards in 90-minute blocks gives bedtimes that end on a lighter note.
What is the Sleep Calculator?
Sleep runs in roughly 90-minute cycles, and waking at the end of a cycle usually feels easier than waking mid-cycle. This tool subtracts 4, 5, or 6 cycles plus about 15 minutes to fall asleep from your chosen wake time, giving three bedtimes to pick from. The result is general information only, not medical advice, and it cannot fix poor sleep quality or a medical sleep problem.
How to use it
- Enter the hour you want to wake up (0 to 23).
- Enter the minute you want to wake up (0 to 59).
- Pick one of the three bedtimes for 4, 5, or 6 sleep cycles.
Worked example
Waking at 7:00 gives bedtimes of 21:45 for 6 cycles, 23:15 for 5 cycles, and 00:45 for 4 cycles.
What time should I go to bed if I wake at 7 am?
Counting back from 7:00 am in 90-minute cycles plus 15 minutes to fall asleep gives 11:15 pm for five cycles, 9:45 pm for six cycles, and 12:45 am for four cycles. Most adults do well on five or six cycles, which is 7.5 to 9 hours in bed. Pick the bedtime you can keep consistently rather than the longest one you cannot.
Frequently asked questions
Why 90-minute cycles?
Sleep moves through light, deep, and dream stages in cycles averaging about 90 minutes, so whole cycles tend to end in lighter sleep.
What if none of the bedtimes suits me?
Pick the closest one and shift your wake time slightly, or aim for the nearest whole number of cycles rather than forcing an awkward bedtime.