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Sleep Cycle Calculator

Recommended bedtimes or wake times based on 90-minute sleep cycles, plus a 15-minute fall-asleep buffer.

Includes 15 minutes to fall asleep.

How it is calculated

Sleep is modelled as repeating 90-minute cycles, plus 15 minutes to fall asleep. For every option the tool uses offset = 15 + cycles × 90 minutes and offers 6, 5, 4 and 3 cycles (9, 7.5, 6 and 4.5 hours of sleep). If you choose a wake-up time, it subtracts the offset to give bedtimes; if you choose a bedtime, it adds the offset to give wake-up times. Times wrap past midnight on a 24-hour clock.

The 5-cycle option is highlighted as the default. The 90-minute length and the 15-minute delay are fixed averages. Real cycles vary between people and through the night, and falling asleep may take you more or less time, so treat the times as a guide and adjust to how you feel.

Worked example

You must wake at 07:00. With 6 cycles the offset is 15 + 6 × 90 = 555 minutes (9 h 15 min), so go to bed at 21:45. With 5 cycles it is 465 minutes (7 h 45 min), giving bedtime 23:15 and 7.5 hours of sleep. With 4 cycles it is 375 minutes, giving 00:45, and with 3 cycles 285 minutes, giving 02:15 for 4.5 hours of sleep.

Try this example →

Common mistakes

  • Going to bed at the listed time instead of getting into bed with the lights off; the 15 minutes to fall asleep are already included, so do not add it twice.
  • Choosing the wrong direction: I want to wake up at returns bedtimes, while I'm going to bed at returns wake-up times.
  • Reading the 02:15 option as healthy; 3 cycles is only 4.5 hours.
  • Expecting exactly 90 minutes per cycle. It is an average and varies from person to person.

Sources

About this calculator

This tool schedules bedtimes or wake-up times around the body's natural 90-minute sleep cycle, the recurring pattern of light sleep, deep sleep and REM that repeats through the night. Waking up mid-cycle, especially during deep sleep, tends to produce grogginess ("sleep inertia"), while waking near the boundary between cycles usually feels much easier, even with the same total number of hours slept.

It is used by anyone trying to time an alarm more intelligently, students and shift workers planning naps or early mornings, and parents working backward from a wake-up time to find the best bedtime for a child. The calculator also adds roughly 15 minutes to account for the time it typically takes to actually fall asleep, so the suggested times reflect when to actually get into bed rather than just when to close your eyes.

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Comments (1)

  • Editorial team

    The bedtimes assume you fall asleep in exactly 15 minutes, so treat them as the time the lights go out. If you usually need 30 minutes, go to bed 15 minutes earlier than the list suggests. The 3-cycle row, 4.5 hours, is a bare minimum for a bad night rather than a target, and the 90-minute cycle is only an average. How long do you usually take to fall asleep? Tell us what changed when you shifted the plan.

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