Sleep Calculator
The evening question runs in reverse: alarm set for 7:30 — when should the lights go out? This calculator counts backwards from your wake time by the sleep you want and returns the bedtime. It repeats the sum rounded to whole 90-minute cycles — an adult cycle averages that length — because many find waking between cycles easier than surfacing mid-sleep. The cycle figure is a scheduling heuristic, not medical advice.
Enter values
Results
- Bedtime
- 23:30
- Bedtime by 90-minute cycles
- 00:00
- Cycles your target sleep rounds to
- 5 cycles = 7 h 30 min
How is the Sleep calculated?
Enter the wake hour and minute, then the sleep you want, in half-hours anywhere from 4 to 12. The plain bedtime row subtracts that duration from your wake time and wraps into the previous day whenever it would cross midnight. The second row redoes the arithmetic with the nearest whole number of 90-minute cycles, and the last row names that cycle count together with the duration it actually buys — five cycles, for instance, buying seven and a half hours.
Formula
Bedtime = (wake time − sleep duration) mod 24 hours. Cycle count = round(sleep minutes ÷ 90); the cycle-aligned bedtime uses cycles × 90 minutes instead. Wake time is the clock at rising, duration the target sleep; the modulo keeps answers inside 00:00–23:59. The 90-minute figure is an adult scheduling average, not a measurement of anyone's stages.
Example
Alarm at 07:30 with eight hours planned: 07:30 minus 8 hours is 23:30, the plain bedtime. Eight hours is 480 minutes, which rounds to five 90-minute cycles totalling 7 h 30 min, and five cycles before 07:30 lands exactly on 00:00 — so the 23:30 sleeper and the midnight sleeper wake with the same five cycles behind them. Ask for nine hours instead and the plain bedtime moves to 22:30 while the cycle row holds six cycles, or 9 h.
Things to remember
Bedtime arithmetic solves the arithmetic; real sleep answers more to routine, light and caffeine than to any number. Treat the cycle alignment as an experiment — run both bedtimes for a week and compare the mornings. Trouble falling or staying asleep night after night is worth raising with a doctor, not a calculator.
Frequently Asked Questions
What is the 90-minute sleep cycle rule?
Adults move through light, deep and REM sleep in roughly 90-minute loops, so the rule rounds a target sleep to whole cycles and aims to end one just before the alarm. It is a popular scheduling heuristic — real cycle lengths vary between people and across the night.
How many hours should an adult sleep?
Most guidance lands on seven to nine, and the calculator's 4-to-12-hour range covers the whole spread with half-hour steps. Needs drift slightly lower with age, though the honest test remains how you feel after seven uninterrupted hours rather than any table.
Is it better to wake at the end of a cycle?
Many people report groggier wake-ups from mid-cycle sleep, which is why the cycle-aligned row exists — it lands you nearer the end of a loop. Evidence for the effect is soft, so treat the second bedtime as an option to try rather than a promise.
Does the calculator count the time to fall asleep?
No — it counts from the moment you intend to be asleep, while most people need a quarter of an hour or so to drop off. If the target is to be asleep rather than in bed by the shown time, move your wind-down routine the same quarter of an hour earlier.
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