Sleep Calculator
Plan the night either way round: enter the time you get into bed to see when four, five or six sleep cycles would have you waking up, or set your alarm and see which bedtimes get you there.
Enter your details above — results update as you type.
Formula and calculation
Worked example
Go to bed at 22:45 and give yourself 15 minutes to fall asleep: four cycles have you waking at 05:00 (Sat), five at 06:30 (Sat) and six at 08:00 (Sat) — that is 6, 7.5 and 9 hours of actual sleep. Waking at the end of a cycle is easier than surfacing in the middle of one.
Why 90 minutes is only a rule of thumb
A sleep cycle moves through light sleep, deep sleep and REM, then starts again, and the average lap takes about 90 minutes. Your own cycles are probably not that tidy: the National Heart, Lung, and Blood Institute puts the range at roughly 80 to 100 minutes, some people run closer to 110, and the last cycles before morning contain more REM and are often shorter. What the arithmetic really buys you is a bedtime that does not land in the middle of deep sleep, which is where waking up feels worst. Total sleep still matters more than the exact minute you surface, so if a plan costs you an hour of sleep to hit a cycle boundary, take the hour.
What matters more than the exact minute
Common sleep-hygiene advice is unglamorous and effective: keep one wake-up time seven days a week, get daylight in the first hour of the morning, keep the bedroom dark and cool, and treat the bed as a place for sleeping rather than scrolling. Caffeine is worth watching — its effects last for hours, so an afternoon coffee can still be working at bedtime — and a late workout or a heavy meal can push sleep onset out by more than the 15 minutes this calculator assumes. If you regularly take much longer than half an hour to fall asleep, or you wake up exhausted no matter how long you are in bed, that is a conversation with a clinician rather than a scheduling problem.
What the results mean
- 4 cycles — wake up at
- Four cycles from the time you gave — the clock time to wake up at, or to go to bed at. A time that lands on another day carries that day's short name.
- 5 cycles — wake up at
- Five cycles, which is 7.5 hours asleep and the plan most adults land on. Same direction as the other two rows.
- 6 cycles — wake up at
- Six cycles, the top of the 7 to 9 hour range most adults need.
- Time asleep with 4 cycles
- Hours actually spent asleep in the four-cycle plan — six hours. The 15 minutes spent falling asleep is not counted as sleep.
- Time asleep with 5 cycles
- Hours asleep in the five-cycle plan: 7.5. This is a common target because it lands in the middle of the recommended range.
- Time asleep with 6 cycles
- Hours asleep in the six-cycle plan: nine. Useful after a short night, but hard to fit in every day.
Frequently asked questions
How many hours of sleep do I need?
Most adults need 7 to 9 hours, the range published by the National Sleep Foundation and the CDC. Teens need 8 to 10 hours and school-age children about 9 to 11. Five cycles is 7.5 hours, which is why it is such a common target — but staying inside the range matters more than any exact number.
Why does it add 15 minutes before counting cycles?
Because the cycles only start once you are asleep. Timing from the moment you get into bed would have you waking about 15 minutes early. Fifteen minutes is the usual estimate for healthy adults, who take roughly 10 to 20 minutes to drop off; if you know you take longer, go to bed that much earlier than the plan says.
Do I have to wake up at the end of a cycle?
No. Waking between cycles is a nicety rather than a rule: total sleep matters more, and if you sleep through the night without trouble the cycle math adds little. It helps most when you expect to feel groggy, when you nap, or when you have to set an alarm for an unusual hour.
Which clock does the calculator use?
Your own clock. The plans are pure arithmetic on the bedtime or wake-up time you give it, so the answers come back in the same terms. Leave the bedtime blank and it defaults to your device's local time, the same time your phone shows.