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Sleep calculator
What it does
This sleep calculator works back from your alarm rather than forward from when you feel tired, because the alarm is the fixed point and the body clock learns the morning. Put in your wake-up time, the hours you're aiming for and how long you take to drop off, and it gives you lights-off and when to start winding down.
- Put in your weekday alarm The one work sets, not the one you would like. The alarm is the fixed point.
- Pick the hours and your drop-off time Seven and a half if you do not know. If you often lie there, allow 30 and treat that as its own problem.
- Hold the alarm for seven mornings Judge it by the afternoons after a fortnight, not by the first morning.
Most sleep calculators ask when you're going to bed and tell you when to get up. That's the sum done backwards. The morning is usually decided for you, by work or the school run.
So the useful question is the other way round. Given the alarm, when does the lamp go off?
How to use it
Put in your weekday alarm, the one work sets rather than the one you'd like. Pick the hours you're aiming for. The NHS puts adult sleep need at seven to nine, and if you don't know where you fall, start at seven and a half.
Then say how long you usually take to drop off. If you often lie there, allow 30 minutes, and see that as a separate problem to solve.
The calculator gives you three times. Lights off: the alarm minus your hours minus your drop-off time. Start winding down: an hour before that, because the NHS suggests at least an hour of relaxing before bed.
And asleep by. That's the time the hours are counted from.
If you'd rather start from a bedtime, flip it round. It gives you the alarm for six, seven and a half or nine hours. It will nudge you back, though: if tomorrow's alarm is already set, the bedtime is what's left.
What the cycle view means
Under the main answer there's a table in 90-minute cycles. Four cycles is six hours, five is seven and a half, six is nine. A cycle is one round of light, deep and dreaming sleep.
The popular idea is that waking at the end of one avoids the grogginess.
Ninety minutes is an average, not a measurement of you. Harvard's sleep division puts the first cycle at 70 to 100 minutes and later ones at 90 to 120. Your own change with age, stress and how you slept last night.
A 2019 review of sleep inertia, the name for that grogginess, found the evidence on whether the stage of sleep at waking matters is mixed. So the cycle sum checks the hours. It can't pick the minute you'll wake.
Why the alarm is the fixed point
Your body clock doesn't read bedtimes. It reads light. Most of all, the light that reaches your eyes after waking.
Your wake-up time decides when that arrives, so it decides where the clock is set.
Bedtime follows from it. It rarely works the other way round. In a 2024 study of 60,977 UK adults, how regular people's sleep was predicted their health better than how long they slept.
That's why the calculator ends with a line about Saturday. Keep the weekday alarm at the weekend if you can. If that's more than you can face, an hour later at most; a longer lie-in moves the clock, and Sunday night runs late.
What it can't tell you
How you'll feel when the alarm goes. That's decided by the week before it. Not by the night.
And if the sum says 10.45pm and you're never asleep before 1am, the problem isn't the arithmetic. It's sleep pressure or the body clock, and how to fall asleep and stay asleep is the place to start.
The sum is worked through by hand in what time should I go to bed?. The case for the alarm is in why your wake-up time matters more than your bedtime. Nothing you type here leaves your browser.
Read next
The evening
What time should I go to bed? Work back from your wake-up time
The evening
Why your wake-up time matters more than your bedtime
The next day
How to wake up feeling refreshed
Questions
How many hours of sleep do you need?
The NHS says seven to nine for adults. Nobody can tell you where in that range you sit except a fortnight of afternoons. If you're flat by 3pm on seven hours, try seven and a half. Judge it by the day, not the first morning.
Is it better to sleep in 90-minute cycles?
Only as a way of checking the hours. Cycles run 70 to 120 minutes and vary night to night, so an alarm set to the end of a cycle is a guess. A fixed wake-up time and enough hours before it do more for the morning.
What time should you go to bed if you wake up at 6.30am?
For seven and a half hours asleep and 15 minutes to drop off, lights off at 10.45pm and winding down from 9.45pm. For eight hours, lights off at 10.15pm. Go up when you're sleepy, not when the clock says; some nights that's later, and that's fine.
Sources
- Insomnia, NHS, 2024
- Fall asleep faster and sleep better, NHS Every Mind Matters, 2026
- Science of sleep: what is sleep?, Harvard Medical School Division of Sleep Medicine, 2021
- Sleep inertia: current insights, Nature and Science of Sleep, 2019
- Sleep regularity is a stronger predictor of mortality risk than sleep duration: a prospective cohort study, Sleep, 2024
This article is general information, not medical advice. If sleep problems last more than a few weeks, speak to your GP.
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