About this sleep calculator
Sleep runs in cycles of roughly 90 minutes. Waking at the end of one feels easy; waking in the middle of one feels awful, even after a long night. This works backwards from the time you need to be up and gives you the bedtimes that land on a cycle boundary.
How to use it
- Enter the time you have to be awake.
- Pick one of the bedtimes it suggests — the later ones are shorter nights.
- Aim for five or six cycles: seven and a half or nine hours.
- It adds fifteen minutes for falling asleep, so go to bed at the time shown, not later.
How long is a sleep cycle?
About 90 minutes on average, moving through light sleep, deep sleep and REM. It varies between people and across the night, so treat these times as a good target rather than a precise measurement.
How many cycles do I need?
Most adults do best on five or six — seven and a half to nine hours. Four is a short night you can manage occasionally; three is a last resort.
Why add time for falling asleep?
Almost nobody is asleep the moment they lie down. Fifteen minutes is the usual estimate, so the bedtimes here are when to be in bed with the light off.
Does going to bed at the same time matter?
More than the exact number of hours. A regular bedtime and wake time is what keeps the cycle predictable, which is what makes waking up easy.