Sleep Calculator
Enter the time you need to wake up and see when to go to bed to wake feeling rested.
Results
- Bedtime for 6 cycles (9 h of sleep)
- Bedtime for 5 cycles (7.5 h of sleep)
- Bedtime for 4 cycles (6 h of sleep)
- Bedtime for 3 cycles (4.5 h of sleep)
A sleep cycle lasts about 90 minutes; waking at the end of one feels easier. Most adults are advised to sleep 7–9 hours (5–6 cycles).
Sleep runs in cycles of about 90 minutes, each moving through light sleep, deep sleep and REM. An alarm that goes off in the middle of deep sleep leaves you groggy; waking at the end of a cycle feels far easier.
The calculator counts 90-minute cycles back from your wake-up time and adds the time it takes you to fall asleep. For most adults five or six cycles — 7.5 to 9 hours — is ideal.
How to calculate Sleep
- Choose whether to calculate from your wake-up time or your bedtime.
- Enter the hour and minute.
- Enter how long you usually take to fall asleep and read the suggested times.
Sleep formula
Bedtime = wake-up time − (cycles × 90 min) − time to fall asleepWake-up time = bedtime + time to fall asleep + (cycles × 90 min)
Sleep example
To wake at 07:00 when you take 15 minutes to fall asleep, go to bed at 23:15 for five cycles (7.5 hours) or at 21:45 for six cycles (9 hours).
Sleep: common questions
How many hours of sleep do I need?
Adults are advised to get 7–9 hours. Teenagers need 8–10 and school-age children 9–11.
Is a sleep cycle always 90 minutes?
It averages 90 minutes but ranges from about 70 to 110 between people. If the suggested time does not feel right, shift it by 15 minutes and try again.
Is 6 hours of sleep enough?
It is four cycles and fine once in a while, but regularly sleeping under 7 hours affects attention, immunity and mood.