Sleep Time Calculator
Human sleep runs in 90-minute cycles (light → deep → REM). Waking at the end of a cycle feels far more refreshed than waking in deep sleep. The tool also adds the 14-minute average fall-asleep time.
How to Use
Choose "If I sleep now, when should I wake?" or "To wake at this time, when should I sleep?".
Enter your reference time. The "Now" button auto-fills the current time.
Six suggested times appear as cards. The 5–6 cycle range (7.5–9h) for adults is highlighted as recommended.
FAQ
Why 90-minute cycles?
Sleep cycles through non-REM (light → deep) → REM about every 90 minutes. Waking from deep sleep produces "sleep inertia" (groggy 1–2 hours), but waking near a cycle boundary (light/REM) feels much more refreshed for the same total sleep time.
Why add 14 minutes?
The average person takes 10–20 minutes to fall asleep (sleep onset latency). 14 minutes is a research-based middle value. If you fall asleep instantly, subtract that buffer mentally.
How many cycles are best?
5–6 cycles (7.5–9 hours) are typical for adults. 4 cycles (6h) is short; 7 cycles (10.5h) can feel heavy. The recommended range is highlighted.
Naps too?
Yes — short naps (20–30 min, light sleep only) and full-cycle naps (90 min) are most refreshing. ~60 min naps are worst because you wake from deep sleep.
Is anything sent to a server?
No. All calculations run in your browser; nothing leaves the page.
