How 90-minute sleep cycles work
During the night you move through repeating cycles of light sleep, deep sleep and REM sleep. Each cycle lasts roughly 90 minutes, although real cycles vary between about 70 and 120 minutes from person to person and across the night. Waking at the end of a cycle, during light sleep, usually feels easier than being pulled out of deep sleep.
- Early cycles contain more deep sleep, which supports physical recovery.
- Later cycles contain more REM sleep, linked to memory and mood.
- Five to six cycles (7.5 to 9 hours) suits most adults; health bodies recommend at least 7 hours a night.
Why the calculator adds 14 minutes
Most healthy adults take around 10 to 20 minutes to fall asleep. The calculator adds 14 minutes so the cycles start when you are actually asleep, not when you get into bed. If you usually fall asleep faster or slower, adjust the setting.
Bedtimes for common wake-up times
| Wake up at | 6 cycles (9 h) | 5 cycles (7.5 h) | 4 cycles (6 h) |
|---|---|---|---|
| 6:00 AM | 8:46 PM | 10:16 PM | 11:46 PM |
| 6:30 AM | 9:16 PM | 10:46 PM | 12:16 AM |
| 7:00 AM | 9:46 PM | 11:16 PM | 12:46 AM |
| 7:30 AM | 10:16 PM | 11:46 PM | 1:16 AM |
| 8:00 AM | 10:46 PM | 12:16 AM | 1:46 AM |
Power naps
A 10–20 minute nap boosts alertness without entering deep sleep. Around 30 minutes may leave you briefly groggy. A full 90-minute nap completes one cycle. Avoid long naps late in the day if you struggle to fall asleep at night.
This calculator is a planning aid, not medical advice. Persistent tiredness, loud snoring or trouble sleeping is worth discussing with a doctor.