How the bedtime calculator works
The 90-minute night
Sleep moves through ~90-minute cycles of light sleep, deep sleep, and REM. Waking mid-cycle — especially from deep sleep — produces sleep inertia, the heavy grogginess that coffee barely touches. Waking at a boundary, when sleep is light, is measurably easier. The calculator works backwards from your alarm, adding a typical 15-minute sleep latency.
Why the hour counts matter more than the minute
Cycle length varies (80–110 minutes) and drifts through the night, so the math is a targeting system, not a guarantee. Its power shows in the difference between 7.0 hours (5 cycles minus most of one) and 7.5 (five complete): same ballpark duration, materially different wake-up experience. Choose cycle counts; adjust by feel.
Consistency is the multiplier
The circadian system rewards regularity above all: same bedtime and wake time daily (yes, weekends within an hour) improves both sleep quality and morning alertness more than any single night's optimization. Pick your cycle count, anchor the wake time, and let the bedtime calculator place the evening.