Sleep Calculator
Line up your alarm with the end of a sleep cycle so you wake between cycles instead of in the middle of one.
| Go to bed at 9:45 PM | 6 cycles, 9 hours of sleep |
|---|---|
| Go to bed at 11:15 PM | 5 cycles, 7.5 hours of sleep |
| Go to bed at 12:45 AM | 4 cycles, 6 hours of sleep |
| Go to bed at 2:15 AM | 3 cycles, 4.5 hours of sleep |
How to use it
- Choose whether you know your wake-up time or your bedtime.
- Enter the time and how long you usually take to fall asleep.
- Most adults need 7 or more hours, so 5 or 6 cycles is the usual target.
Formula
One sleep cycle ≈ 90 minutes (an average; real cycles vary)Bedtime = wake time − cycles × 90 min − time to fall asleepWake time = bedtime + time to fall asleep + cycles × 90 min
Example
To wake at 7:00 AM, taking 15 minutes to fall asleep, go to bed at 9:45 PM for 6 cycles (9 hours of sleep) or 11:15 PM for 5 cycles (7.5 hours). Going to bed at 11:00 PM, good wake-up times are 6:45 AM (5 cycles) or 8:15 AM (6 cycles).
Questions
Are sleep cycles really 90 minutes?
About that on average, but they vary from person to person and through the night. Treat the times as a starting point.
How much sleep do adults need?
Adults are generally advised to get 7 or more hours a night. If you're always tired, talk to a doctor.
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Last reviewed 2026-10-11. Found a mistake? Tell us.