Sleep Cycle Calculator
Ninety-minute cycles, plus 15 minutes to fall asleep. Waking at the end of a cycle is what makes the difference.
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Worked example
To wake at 07:00 after six cycles, go to bed at 21:45 — 540 minutes of sleep plus 15 to fall asleep. Five cycles moves that to 23:15.
Sleep runs in cycles of roughly 90 minutes, moving through light sleep, deep sleep and REM. Waking mid-cycle, especially out of deep sleep, produces grogginess that can last an hour even after adequate total sleep.
Why 90 minutes is approximate
Cycle length varies between 70 and 120 minutes depending on the person and the night, and shifts toward longer cycles later in the night. The calculator gives a target, not a guarantee; the useful signal is that going to bed 20 minutes later can feel worse than sleeping less.