The 90-minute cycle
Sleep is organized into cycles of about 90 minutes each: light sleep → deep sleep → REM → back to light. Waking up between cycles (during light sleep) leaves you refreshed; waking mid-deep-sleep causes that groggy "sleep inertia" that lingers for hours.
How to use this
- Fixed wake time? Count back 90-minute cycles (aim for 5–6 → 7.5–9 hours) and go to bed at one of those times. Include ~14 minutes to fall asleep.
- Sleeping now? Set your alarm for a multiple of 90 minutes from now plus about 15 minutes to fall asleep — 6 cycles (9h) or 5 cycles (7.5h) are the sweet spots.
- Consistency beats perfection: same bedtime and wake time daily anchors your circadian rhythm better than any cycle math.
Caveats
Cycles vary from person to person (80–110 min) and change across the night (later cycles have more REM). This is a helpful planning heuristic, not a precision instrument — combine it with how you actually feel.