Each sleep cycle is ~90 minutes. Waking at the end of a cycle — rather than mid-cycle — helps you feel less groggy. Plan 5–6 cycles (7.5–9h) for a full night.
How it works
Sleep cycles through light, deep and REM phases roughly every 90 minutes. Waking mid-cycle (especially during deep sleep) is what makes you feel groggy even after enough hours. This calculator works backwards (or forwards) in 90-minute cycles, plus a few minutes to actually fall asleep.
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