In more detail
Each cycle passes through light sleep (stages N1 and N2), deep slow-wave sleep (N3) and REM sleep, when most vivid dreaming happens. Early cycles contain more deep sleep, while later ones contain more REM. Five cycles are 7.5 hours (plus the roughly 15 minutes it usually takes to fall asleep). Cycle lengths vary between people and through the night, from about 70 to 120 minutes, so treat 90 minutes as an average.
Why it matters
Waking from deep sleep tends to cause grogginess (sleep inertia), which is why bedtime calculators aim for the end of a cycle. They are estimates, not measurements: total sleep matters more than hitting a cycle boundary exactly, and health bodies generally recommend at least 7 hours a night for adults. Persistent tiredness despite enough time in bed is worth discussing with a doctor.
Example
To wake at 7:00, counting back whole 90-minute cycles plus 15 minutes to fall asleep gives these bedtimes:
- 6 cycles (9 h): 21:45
- 5 cycles (7.5 h): 23:15
- 4 cycles (6 h): 00:45