Sleep Cycle Calculator — Free Online Tool

Sleep happens in ~90-minute cycles, and waking up mid-cycle (during deep sleep) is what makes mornings feel rough — waking up between cycles feels much easier, even with less total sleep. Pick a wake-up time or a bedtime and this tool works backward or forward through whole 90-minute cycles, plus a fixed buffer to actually fall asleep. Everything runs locally; nothing is sent anywhere.

Wake-up time

time to fall asleep minutes

Suggested bedtimes, counting back in whole 90-minute cycles from your wake-up time:

A full cycle moves through light → deep → REM sleep and back to light, where it's easiest to wake up. Most adults do best on 5–6 cycles (7.5–9 hours); rows within that range are marked ★. Actual cycle length varies person to person (roughly 80–110 min), so treat this as a starting point, not a guarantee.

Power nap timer

If it's right now, napping this long gets you these wake times — short naps stay in light sleep, a full 90-min nap completes a cycle and skips grogginess.