How it is calculated
Sleep runs in cycles of about 90 minutes (light, deep, then REM). Waking mid-cycle, especially from deep sleep, causes grogginess.
Bedtime = wake time − (cycles × 90 min) − time to fall asleepAdults do best with 5–6 cycles (7.5–9 hours); we allow 15 minutes on average to fall asleep.
Worked example
To wake at 6:30 after 6 cycles: 6:30 − 9 h − 15 min = bed at 21:15, or 22:45 for five cycles.
Frequently asked questions
How much sleep do adults need?
Health bodies recommend 7–9 hours for adults, more for teens and children.
Is a cycle always 90 minutes?
It ranges from about 80 to 110 minutes and varies between people and through the night, so this is approximate.
Are naps good?
A short 20-minute nap or a full 90-minute cycle; avoid in between.