15 min
Wake up at the end of a cycle — refreshed, not groggy. Free, private, no signup.
Adult sleep is organized into repeating cycles of roughly 90 minutes. Each cycle moves through light sleep → deep (slow-wave) sleep → REM. Waking near the end of a cycle — when sleep is lightest — feels noticeably easier than being jolted out of deep sleep (that heavy “sleep inertia” feeling).
This tool counts complete 90-minute blocks forward or backward from the time you choose and adds a sleep-onset latency buffer (the time it typically takes you to fall asleep after getting into bed). The highlighted rows target 5 or 6 full cycles — the range most adults need for restorative sleep.
Default latency is 15 minutes (typical median for healthy adults). Move the slider if you usually fall asleep faster or slower.
Clinical and research summaries (Sleep Foundation, NHLBI, StatPearls) place the average adult sleep cycle near 90 minutes. Individual cycles commonly range 80–110 minutes, and the first cycle of the night is often shorter and deeper while later cycles contain more REM. Treat the results as a strong starting point and fine-tune by how you actually feel after a few nights.
It counts complete 90-minute cycles and adds your fall-asleep buffer so the alarm lands near the end of a cycle when sleep is lightest.
No. This is a planning tool based on population averages. Persistent insomnia, loud snoring, gasping, or excessive daytime sleepiness should be discussed with a clinician.
Yes. Enter whatever wake time or bedtime you need. The cycle math still applies. The harder parts of shift work are consistency, light exposure, and recovery days.
If you get into bed at 10:30 but only fall asleep at 10:45, your first cycle starts at 10:45. The buffer makes the calculation land on actual sleep time rather than “lights-out” time.
No. Everything runs in your browser. Times and settings are never uploaded. Optional local storage only remembers your last preference on this device.