Sleep Calculator

Working in ninety-minute cycles and allowing a quarter of an hour to drop off, the good candidates are 21:45, 23:15 and 00:45 — five, six or four cycles. The list on the page does this for whatever time you actually need, and the top of the list is the one that gives you the most sleep rather than the least.

On average, roughly. It is not a fixed number: cycles run from about seventy to about a hundred and twenty minutes, vary between people, and lengthen as the night goes on because REM takes a larger share of each round. Ninety is a reasonable middle, which is why every calculator uses it — but this one lets you change it, because presenting an average as a law is the thing wrong with the rest of them.

That is how long it takes an average adult to fall asleep, so a bedtime and a sleep-time are not the same instant. Fifteen is a middle figure — under five minutes consistently can be a sign of being badly short of sleep, and consistently over thirty is one of the things insomnia looks like. Change it to whatever is true for you.

Most adults need somewhere between seven and nine hours, which is five to six cycles. Six cycles is nine hours and more than many people have time for; five is seven and a half and is the one most of these lists are really recommending. Teenagers need more, and “I only need five hours” is usually somebody who has stopped noticing the deficit.

It helps with how you feel in the first half hour. Waking out of deep sleep produces sleep inertia — the heavy, slow, unpleasant twenty minutes that feels like being hungover — and waking from light sleep at the end of a round largely avoids it. What it cannot do is make less sleep into enough sleep: six hours timed perfectly is still six hours.

No. It is arithmetic on a clock, and it knows nothing about you. If you are sleeping badly rather than sleeping awkwardly — lying awake, waking repeatedly, exhausted despite the hours — that is worth raising with a doctor, and no calculator is a substitute for it.