Sleep Cycle Calculator

A full night alternates through light, deep, and REM sleep in cycles averaging about 90 minutes. Waking during light sleep at a cycle boundary feels easy; an alarm landing mid-deep-sleep produces that heavy, disoriented grogginess (sleep inertia). Individual cycles range roughly 80–110 minutes, so treat the suggestions as good targets, not exact science.

Five to six cycles — 7.5 to 9 hours — covers the recommendation for most adults. Four cycles (6 hours) works occasionally but shortchanges REM sleep, which concentrates in the later cycles of the night.

Yes — it adds an average 15 minutes of sleep latency. If you reliably fall asleep faster or slower, shift the suggested times accordingly.

Cycle timing helps with grogginess, not with insufficient or poor-quality sleep. Inconsistent schedules, late caffeine, alcohol, and screen light all degrade sleep regardless of when the alarm rings. Persistent exhaustion despite adequate hours is worth raising with a doctor.