The stages in one cycle
Each ~90-minute cycle runs light sleep (N1/N2) → deep sleep (N3, early cycles dominated) → REM (later cycles dominated). Early-night cycles are deep-heavy (physical restoration); late-night cycles are REM-heavy (memory and mood). Cutting the night short at either end steals a different currency — hours or mornings tell you which.
Why the wake-up moment decides the morning
Waking from deep sleep produces sleep inertia — up to 30 minutes of cognitive fog that coffee only softens. Waking from light sleep or REM, the same total hours feel completely different. This is the entire value of cycle-targeted alarms: not more sleep, but better-located sleep. The bedtime and wake calculators here exist to move the alarm 20 minutes and transform the morning.
The practical system
Anchor your wake time first (it drives the rhythm), count back in 90-minute blocks plus your true latency, and pick the cycle count your schedule supports. Then protect consistency — the circadian system discounts chaotic schedules more than short ones. One alarm, no snooze, light on immediately: the morning triad that lets the math work.