The sleep-cycle formula
Sleep moves in cycles of roughly 90 minutes, and it takes about 15 minutes to fall asleep once you're in bed. To wake refreshed, aim to wake at the end of a cycle:
bedtime = wake time − (cycles × 90 + 15) minutes
wake time = bedtime + 15 + cycles × 90 minutes
The 15 minutes is sleep latency — the time before you're actually asleep. Times are wrapped around a 24-hour clock, so a bedtime calculated past midnight rolls correctly into the evening before.
Worked example
You need to wake up at 7:00 am. Counting backward in cycles plus 15 minutes:
6 cycles (9 h): 7:00 − (540 + 15) min = 9:45 pm.
5 cycles (7.5 h): 7:00 − (450 + 15) min = 11:15 pm.
4 cycles (6 h): 7:00 − (360 + 15) min = 12:45 am.
The stages inside a cycle
| Stage | What happens |
|---|---|
| N1 — light | Drifting off; easy to wake. A few minutes. |
| N2 — light | Heart rate slows, body temperature drops. Largest share of the night. |
| N3 — deep | Slow-wave sleep; physical recovery. Hardest to wake from. |
| REM | Vivid dreaming, memory consolidation. Lengthens toward morning. |
Note: 90 minutes is an average — real cycles range from about 70 to 120 minutes and shift through the night. Treat these times as a guide and adjust to what leaves you feeling rested.