What this does

Human sleep runs in ~90-minute cycles (light → deep → REM → back to light). Waking between cycles feels refreshed; waking mid-cycle — even after more total hours — feels groggy. This calculator picks bedtimes (or wake times) that put you between cycles.

Two modes

• Wake me at… — enter your alarm time. You get 4 suggested bedtimes for 6, 5, 4 or 3 cycles of sleep. Most adults need 5-6.
• Sleep now — starting from the current time (plus a fall-asleep buffer), see 4 wake times matching 3-6 completed cycles.

The default fall-asleep buffer is 14 minutes — the average time a healthy adult takes to drift off. Adjust it if you know yours differs.

Caveats

Cycle length varies by person (roughly 80-110 min). Cycle length also lengthens through the night. This is a heuristic, not sleep-lab precision — but it beats a random alarm.