Sleep calculator
Two questions on one page: when to go to bed, and where the day went.
When should you go to bed?
Tell us when you need to be up. We will work backwards in ninety-minute cycles.
Where your day goes
Mark the hours that are already spoken for. Whatever is left over is the time available for sleep.
Tap a block to edit it. With a block selected, the arrow keys move it, shift and the arrow keys change how long it is, and Delete removes it.
That is below what most adults need.
Your day
No change yetShrink or remove a block to see what it gives back. The outline shows your day as it was.
Your day is saved in this browser only. No account, and nothing is sent anywhere.
How to work out a bedtime in sleep cycles
The interactive tool needs JavaScript. You can do it just as well on your own:
- Choose a wake time. Decide when you need to be up — say 06:40.
- Count back in cycles. Subtract ninety minutes at a time. Five cycles is seven and a half hours; six is nine hours. From 06:40, six cycles takes you to 21:40.
- Allow time to fall asleep. Most people take ten to twenty minutes to drop off, so go to bed about fifteen minutes earlier: 21:25.
- Treat it as a guide. Ninety minutes is an average, not a rule. Cycles vary between people and from night to night, so use the result as a rough starting point.
How to see where your day goes
The bar needs JavaScript. The arithmetic is simple enough to do on paper:
- Start from midnight. The bar runs from midnight to midnight. Everything you put on it is time already spoken for.
- Add what is fixed. Add work, commute, exercise and meals, each with a start and an end time.
- Add screen time in bed. Add the time you spend on a phone or tablet after getting into bed. It has its own block because it is the one people underestimate.
- Read what is left. The unfilled part of the bar is the time left for sleep.
- Take off the honest minutes. Enter how long you take to fall asleep and how long you are usually awake in the night. What remains is the sleep you actually get.
- Try a change. Use “What if…” to shrink or remove a block and see how many minutes it gives back before you commit to it.
On paper: write down the hours you spend on work, commuting, exercise, meals and screens in bed. Add them up, subtract the total from 24, then take off the time you need to fall asleep and any time you are awake in the night. What is left is the sleep you actually get.
Two ways of looking at the same night
The first tab works forwards and backwards in sleep cycles: tell it when you need to be up, and it counts back in ninety-minute blocks, adding a little time to fall asleep. Ninety minutes is an average, not a rule, so treat the result as a starting point and adjust it over a week or two.
The second tab asks a blunter question. “Bed at midnight, up at six” sounds like six hours, but it rarely is: there is the time it takes to fall asleep, the phone in bed, and the stretch awake at four. Drawing the day as one bar makes the arithmetic visible. It does not score anything or tell you what to do — it just shows you what is left.
Read more in Sleep cycles, and why ninety minutes is only an average.
Further reading
Questions
Is a sleep cycle always ninety minutes?
No. Ninety minutes is an average. Cycles vary between people and through the night, from roughly seventy to a hundred and twenty minutes. Adjust the cycle length under “Adjust the numbers” if you know yours runs shorter or longer.
Why add fifteen minutes?
Most people take somewhere between ten and twenty minutes to fall asleep. The calculator adds that on so the cycles start when you actually drop off, not when you get into bed.
Which time should I pick?
For most adults, five or six cycles — seven and a half to nine hours — is a sensible starting point. The highlighted time is the longest option within that range.
What does the second tab do?
It draws your day as a single 24-hour bar. You add the hours already spoken for — work, commute, exercise, meals, screen time in bed — and whatever is left over is the time available for sleep. It does the subtraction that “bed at midnight, up at six” hides.
Why is screen time in bed its own block?
Because it is the block most people underestimate, and it comes directly out of sleep rather than out of the evening. Giving it its own colour makes that visible.
What if I work nights?
Blocks can cross midnight. Enter a shift that starts at 22:00 and ends at 06:00 and it is drawn in two pieces, at each end of the bar, and counted once.
What does “Set a reminder” do?
It downloads a small calendar file for the time you picked, which your phone or computer offers to add to your calendar. Nothing is sent anywhere.
Does anything I enter leave my device?
No. Both tabs work entirely in your browser, and your day is saved only in this browser’s local storage. There is no account and nothing is sent to a server.
I sleep the hours but still wake up tired. Why?
Waking tired after a full night can have many causes, some worth checking with a doctor — including sleep apnoea, low iron or low mood. See “When to get help”.