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How it works
Sleep runs in cycles that are often approximated as about 90 minutes. This tool counts back 4, 5 or 6 cycles from your wake time and adds about 14 minutes to fall asleep. It is a rule of thumb, not medical advice. The CDC recommends that adults aged 18–60 get 7 or more hours per night.
Source for the 7+ hour guideline: CDC, "About Sleep".
Worked example
To wake at 6:30 AM after five 90-minute cycles (7.5 hours), count back 7 hours 30 minutes to 11:00 PM, then subtract 14 minutes to fall asleep: aim to be in bed by about 10:46 PM. For six cycles (9 hours) it is about 9:16 PM.
How much sleep adults need
The CDC and the American Academy of Sleep Medicine recommend 7 or more hours a night for adults. Cycle length varies from person to person and through the night, so use the times as a starting point and adjust by 15 minutes at a time. Persistent trouble sleeping is worth raising with a doctor.
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Frequently asked questions
How many hours of sleep do adults need?
The CDC recommends 7 or more hours a night for adults aged 18–60.
Why 90 minutes?
A full sleep cycle through light, deep and REM sleep averages roughly 90 minutes, though it ranges from about 70 to 120.
Can I use it for naps?
A 20-minute nap avoids deep sleep; a 90-minute nap covers one full cycle.