How the sleep cycle calculator works
Sleep runs in roughly 90-minute cycles that move from light sleep into deep sleep and finally REM. Waking in the middle of a cycle — especially during deep sleep — triggers sleep inertia, the groggy fog that lingers for up to 30 minutes. The calculator counts backward (or forward) in 90-minute blocks from your target time and adds a fall-asleep buffer, so each suggested time lands at the end of a cycle when you're already in light sleep and easiest to wake.
Wake-up, bedtime, and nap timing in one place
Working an early shift? Use Wake-up mode to see the latest bedtimes that still deliver whole cycles. Heading to bed now? Sleep Now maps your best wake-up times from the current minute. The built-in nap calculator handles the two naps that actually work — a 20-minute power nap that boosts alertness without grogginess, and a single full-cycle recovery nap that includes REM. Everything is calculated locally in your browser, and the fall-asleep latency and cycle length are adjustable if your own data differs from the 90-minute average.
- 01
- Three modes: Wake-up target, Bedtime target, and Sleep-Now
- 02
- 90-minute cycle windows covering 3–6 full cycles
- 03
- Adjustable fall-asleep latency and average cycle length
- 04
- Built-in power-nap and full-cycle recovery-nap calculator
- 05
- Sleep-stage timeline (light, deep, REM) on every result
- 06
- Sleep quality rating: Optimal, Functional, or Sleep Debt Risk
- 07
- 100% local — no sign-up, nothing leaves your device
