CalcPenguin

Sleep Calculator

Pick the time you need to wake up, or the time you're going to bed, and get times that let you wake at the end of a sleep cycle.

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minutes

Go to bed at

10:45 pm 5 cycles, 7.5 hours

For 6 cycles
9:15 pm 9 hours
For 4 cycles
12:15 am 6 hours

To wake at 6:30 am at the end of a cycle, go to bed at 9:15 pm (9 hours of sleep) or 10:45 pm (7.5 hours). 12:15 am gives only 6 hours.

How it's worked out

  1. One sleep cycleabout 90 minutes
  2. Back from your alarm6:30 am − 5 × 90 min − 15 min to fall asleep10:45 pm
  • Cycles vary from about 80 to 120 minutes and change through the night, so treat these times as a guide. Adults need 7 to 9 hours.

How the times are worked out

Bedtime = wake time − cycles × 90 minutes − time to fall asleep

Worked example

To wake at 6:30 am, allowing 15 minutes to fall asleep, go to bed at 10:45 pm for five cycles (7.5 hours) or 9:15 pm for six (9 hours).

Questions people ask

How long is a sleep cycle?

About 90 minutes on average, though cycles run from about 80 to 120 minutes and change through the night.

Why wake at the end of a cycle?

Waking from deep sleep can leave you groggy. Waking near the end of a cycle, in lighter sleep, usually feels easier.

How much sleep do adults need?

7 to 9 hours: five or six cycles. Teenagers need 8 to 10 hours.

Formula and rules last checked 25 October 2026Written and tested by the CalcPenguin teamReport a problem