Sleep Calculator
Find the best bedtimes or wake-up times based on sleep cycles, with a clear account of how much the 90-minute figure can actually be relied on.
Based on 90-minute sleep cycles, plus ~15 minutes to fall asleep. Aim for 5–6 cycles (7.5–9 hours).
The uncertainty overtakes the cycle at the second one
The whole idea is to wake at the end of a cycle rather than in the middle of one, which needs the end of the cycle to be somewhere you can aim at. Cycles run about 70 to 120 minutes, so the possible position of the end spreads by 50 minutes for every cycle that passes.
| After | Spread | Could end between | |
|---|---|---|---|
| 1 cycle | 50 min | 1h 10m – 2h 00m | |
| 2 cycles | 100 min | 2h 20m – 4h 00m | wider than a cycle |
| 3 cycles | 150 min | 3h 30m – 6h 00m | wider than a cycle |
| 4 cycles | 200 min | 4h 40m – 8h 00m | wider than a cycle |
| 5 cycles | 250 min | 5h 50m – 10h 00m | wider than a cycle |
| 6 cycles | 300 min | 7h 00m – 12h 00m | wider than a cycle |
From the second cycle onward the spread is wider than the thing being aimed at. By six cycles it is 300 minutes — the sixth cycle could end anywhere from 7h 00m to 12h 00m after falling asleep, which is more than half the night it was meant to place. Past the first cycle or two, every minute of the night is the end of some cycle for somebody.
The first cycle is the exception, and it is the one worth using. A short nap aimed at twenty minutes, or a single full cycle aimed at ninety, are both short enough that the spread has not yet grown past the thing it is measuring.
Which is fine, because the times are good for a simpler reason
Cycles times 90, plus 15 minutes to fall asleep:
| Cycles | Time in bed | |
|---|---|---|
| 6 | 9h 15m | a reasonable night |
| 5 | 7h 45m | a reasonable night |
| 4 | 6h 15m | short |
| 3 | 4h 45m | short |
Those are good bedtimes for the ordinary reason — they are sensible amounts of sleep spaced an hour and a half apart — rather than for the cycle reason. The five-cycle row is 7h 45m, which would still be a reasonable night if cycles turned out to be eighty minutes or a hundred.
The 3-cycle row is 4h 45m. It is on the list because the arithmetic produces it, not because it is enough sleep — 2 of the 4 options fall under seven hours, and only 2 clear it.
The 15 minutes to fall asleep is its own average, running from about 5 to 30 — a 25-minute spread that lands once, at the front, and shifts every later boundary by the same amount. Smaller than the cycle problem, and it is the one you can actually feel.
How to use
- Enter the time you need to wake up, or the time you are going to bed.
- Read the suggested times.
- Allow time to fall asleep, not just to be in bed.
- Prioritise a consistent schedule over hitting an exact cycle.
Frequently asked questions
What is a sleep cycle?
A progression through light sleep, deep sleep and REM, repeating through the night. The often-quoted 90 minutes is an average — real cycles range from roughly 70 to 120 minutes, vary between people, and change across the night, with deep sleep concentrated early and REM later.
Does waking at the end of a cycle really help?
There is a real effect behind it: waking from deep sleep produces grogginess, called sleep inertia, that can last considerable time. But since cycle length varies and cannot be predicted precisely, timing bedtime to a 90-minute multiple is a rough heuristic rather than a reliable technique.
How much sleep do I need?
Most adults need seven to nine hours, and the proportion who genuinely function well on much less is very small — far smaller than the proportion who believe they do. Need declines somewhat with age but not as much as is commonly assumed.
How long should it take to fall asleep?
Ten to twenty minutes is typical. Falling asleep the moment your head touches the pillow is usually a sign of insufficient sleep rather than a talent. Consistently taking much longer than half an hour is worth discussing with a clinician.
Can I catch up at the weekend?
Partially, and imperfectly. Some recovery is possible, but shifting your schedule by hours at weekends produces an effect resembling jet lag that makes Monday harder. A consistent wake time every day does more for sleep quality than any amount of weekend catching up.
What actually improves sleep?
A consistent schedule, a dark cool room, avoiding screens and bright light late, limiting caffeine after early afternoon, and not lying awake in bed for long periods. Persistent sleep problems warrant a clinician rather than a calculator — nothing here is medical advice.
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