Sleep cycle calculator
You feel most refreshed when you wake at the end of a sleep cycle rather than in the middle of one, when deep sleep makes you groggy. Sleep runs in roughly 90-minute cycles, so this tool finds bedtimes (or wake-up times) that land you at a clean cycle boundary. It is handy for setting an alarm that leaves you alert, or for working out when to turn in tonight to hit a fixed wake-up.
How it works
From your target time the calculator steps in 90-minute blocks and adds a fixed 14 minutes to fall asleep. In “wake up at” mode it counts backward (so each result is a bedtime); in “go to sleep at” mode it counts forward (each result is a wake-up time). It lists 6, 5, 4 and 3 full cycles — 9, 7.5, 6 and 4.5 hours of sleep.
bedtime = wake time − (cycles × 90 min + 14 min)
Worked example
To wake at 07:00:
| Cycles | Total sleep | Go to sleep at |
|---|---|---|
| 6 | 9.0 h | 21:46 |
| 5 | 7.5 h | 23:16 |
| 4 | 6.0 h | 00:46 |
| 3 | 4.5 h | 02:16 |
Most adults feel best on 5–6 cycles. Pick a direction, enter a time as HH:MM, and the list appears instantly — all in your browser, nothing uploaded.
Why cycle boundaries matter
A single sleep cycle moves through four stages. Stage 1 and 2 are light sleep, Stage 3 is deep slow-wave sleep, and REM (rapid eye movement) sleep follows. Early in the night, cycles are dominated by deep sleep; later cycles contain more REM. Waking mid-cycle — especially mid-Stage 3 — triggers sleep inertia: the grogginess, confusion and reduced reaction time that can persist for 15–30 minutes. Waking at the end of a cycle, during the light sleep that closes each cycle, largely avoids this.
This is why two people who slept “the same amount” can feel very different in the morning: one hit the alarm at a cycle boundary, the other was dragged out of deep sleep 45 minutes into a cycle.
Getting more from the results
- Aim for 5 or 6 cycles (7.5–9 hours) for most nights. The 4-cycle (6 hours) option is workable for occasional short nights but tends to accumulate sleep debt across the week.
- The 14-minute fall-asleep offset is an average. If you typically fall asleep faster or slower, adjust your bedtime by that difference.
- Real cycles are not exactly 90 minutes. The first cycles of the night tend to be shorter (closer to 70–80 minutes) and later ones longer (closer to 100–110 minutes). The 90-minute average gives a useful planning target, but the best feedback is how you feel at the alarm — use that to nudge bedtime by 15 minutes in either direction over a few nights.
- Consistency matters more than any single night. Going to bed and waking at the same times every day, including weekends, stabilises your circadian rhythm and generally makes waking feel less brutal regardless of exactly where in the cycle the alarm falls.