Sleep Calculator: Bedtime and Wake-Up Time
When to go to bed, or when to wake up, to fit whole 90-minute sleep cycles.
Good to know. Waking at the end of a sleep cycle feels easier than waking in the middle of one. Hence the odd bedtimes.
Approximate. Real-world values vary.
How it works
In plain words
Sleep runs in cycles of about 90 minutes. To wake up at the end of a cycle, count back from your alarm (or forward from bedtime) in steps of 90 minutes, and allow the time it usually takes you to fall asleep.
The formula
bedtime = wake-up time − time to fall asleep − cycles × 90 min wake-up time = bedtime + time to fall asleep + cycles × 90 min
Worked example
To wake at 07:00 after 6 full cycles, go to bed at 07:00 − 15 min − 9 h = 21:45. For 5 cycles, 23:15; for 4 cycles, 00:45.
Sources
- Hirshkowitz et al., National Sleep Foundation’s sleep time duration recommendations, Sleep Health, 2015 Adults 7 to 9 hours, teenagers 8 to 10, school-age children 9 to 11.
- Carskadon and Dement, Normal human sleep: an overview, in Principles and Practice of Sleep Medicine Sleep cycles of roughly 90 minutes through the night.
Sources checked September 30, 2026. 3 automated tests check its answers on every change.
About the sleep calculator: bedtime and wake-up time
While you sleep, your brain moves through cycles of light sleep, deep sleep and dream (REM) sleep. Each cycle lasts roughly 90 minutes. Waking at the end of a cycle, when sleep is lightest, tends to feel easier than being pulled out of deep sleep by an alarm, even if the total time asleep is a little shorter.
Choose whether you have a fixed wake-up time or a fixed bedtime. The calculator counts 90-minute cycles back from the alarm, or forward from bedtime, and adds the time it usually takes you to fall asleep. Most people take 10 to 20 minutes; set it to what is normal for you.
How much sleep do you need? Adults are generally advised to get 7 to 9 hours a night, which is five or six cycles. Teenagers need 8 to 10 hours and school-age children 9 to 11. Four cycles (6 hours) now and then won’t hurt, but regularly sleeping that little is linked to poorer concentration, mood and health.
Treat the times as a guide. Cycles vary from person to person and get longer and lighter toward morning, so the 90-minute rule is an average. If you regularly wake tired after enough hours, or snore heavily, talk to a doctor: sleep problems are common and often treatable.
Questions people ask
What time should I go to bed to wake up at 7?
For 6 cycles (9 hours), around 21:45; for 5 cycles (7.5 hours), around 23:15, allowing 15 minutes to fall asleep.
How long is a sleep cycle?
About 90 minutes on average, though it varies between people and through the night.
How many hours of sleep do adults need?
Most adults need 7 to 9 hours a night, according to the US National Sleep Foundation.