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Sleep Calculator

When should you go to bed so the alarm doesn't go off in the middle of deep sleep? Tell it when you have to get up — or when you're heading to bed.

You should fall asleep at:

  1. 21:45 6 cycles · 9 hours of sleep
  2. 23:15 5 cycles · 7 hr 30 min of sleep
  3. 00:45 4 cycles · 6 hours of sleep
  4. 02:15 3 cycles · 4 hr 30 min of sleep

Counted in 90-minute cycles with 15 minutes to fall asleep. Five to six cycles is the usual guide for adults.

And for the minutes before that: a Bible verse to fall asleep to.

Why count in cycles rather than hours

Sleep isn't uniform. It moves through stages — light sleep, deep sleep, REM — and repeats that sequence several times a night. One full pass averages about 90 minutes. When the alarm lands at the end of a cycle, in light sleep, getting up is usually easier. When it drags you out of deep sleep you feel foggy, even if you slept longer overall. That is why a shorter night can leave you feeling better than a longer one.

90 minutes is an average, not a stopwatch

Here is what most sleep calculators leave out: cycles are individual. They run roughly 70 to 120 minutes, differ from person to person, and lengthen as the night goes on — deep sleep gives way to more REM. A calculator that names your bedtime to the minute is claiming to know something nobody knows.

So treat these as suggestions. Try them over a few nights, find the one that leaves you rested, and then keep to it. A consistent rhythm does more than any single perfectly-calculated night.

When the problem isn't the timing

A calculator helps you plan. It does not help when you are lying there and your thoughts will not settle. We have written something more useful for that:

Frequently asked questions

How does the sleep calculator work?
Tell it when you have to get up. It counts back in sleep cycles of about 90 minutes and adds 15 minutes for falling asleep, then offers you several bedtimes. You tend to wake more easily when the alarm lands at the end of a cycle rather than in the middle of deep sleep.
Is a sleep cycle exactly 90 minutes?
No, and no calculator can make it so. Ninety minutes is an average. Real cycles run roughly 70 to 120 minutes, differ from person to person, and get longer as the night goes on. Treat the times as starting points rather than a stopwatch — the right one is the one you wake up feeling rested after.
How many sleep cycles do I need?
Five or six is the usual guide for adults, which is about seven and a half to nine hours. Four cycles (six hours) is a short night you can get away with occasionally. Three is not enough for more than a day.
Why add 15 minutes for falling asleep?
Because almost nobody is asleep the moment the light goes off. Ten to twenty minutes is normal. If it reliably takes you much longer, move your bedtime earlier by the difference.
Does a consistent schedule matter more than hitting a cycle?
Yes. Getting up at roughly the same time every day does more than calculating any single night. Use the calculator to find a rhythm, not to optimise one bedtime.

Still reads scripture aloud at bedtime — evening prayers, devotionals, psalms and sleep stories, so the wait before sleep isn't the hardest part of the day. Get the app.

This calculator is a rough guide, not medical advice. If you sleep badly over a long stretch, talk to your doctor.