How to use the Sleep Calculator
- Choose whether you know your wake-up time or your bedtime.
- Enter the time and how long you usually take to fall asleep.
- Pick one of the suggested times. Five or six cycles suit most adults.
How it works
Sleep runs in cycles of light sleep, deep sleep and REM sleep. A cycle lasts roughly 80 to 110 minutes, and 90 minutes is a common average. Waking at the end of a cycle, when sleep is lighter, usually feels easier than waking from deep sleep.
Health bodies including the American Academy of Sleep Medicine and the CDC recommend at least 7 hours a night for adults. Five cycles (7.5 hours) and six cycles (9 hours) fit that range, so those times are highlighted. Treat the times as a guide: your own cycles may be a little shorter or longer.
Formula
Bedtime = wake-up time − cycles × 90 min − time to fall asleep Wake time = bedtime + time to fall asleep + cycles × 90 min
Example
To wake at 6:30 AM, taking 15 minutes to fall asleep, go to bed at 10:45 PM for five cycles or 9:15 PM for six.
Frequently asked questions
What time should I go to bed to wake up at 7 AM?
Around 11:15 PM for five cycles (7.5 hours) or 9:45 PM for six cycles (9 hours), if you take about 15 minutes to fall asleep.
Is 6 hours of sleep enough?
For most adults, no. The recommendation is at least 7 hours. Regularly sleeping 6 hours or less is linked with poorer concentration and health.
Why do I feel groggy after a full night's sleep?
Waking from deep sleep can cause sleep inertia, a heavy feeling that usually fades within 15 to 30 minutes. Daylight and a regular schedule help.
Are sleep cycles always 90 minutes?
No. They vary between people and through the night, with more deep sleep early and more REM sleep towards morning.
Last reviewed October 20, 2026. Spotted an error? Tell us.