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Sleep Calculator

When to go to bed or set your alarm so you wake at the end of a sleep cycle, not halfway through one.

Runs entirely in your browser. Nothing you enter is uploaded or stored.

To wake up at 7:00 AM at the end of a sleep cycle, go to bed at one of these times:

  1. 9:45 PM

    Asleep by 10:00 PM

    9 h · 6 cycles

    Best

  2. 11:15 PM

    Asleep by 11:30 PM

    7 h 30 min · 5 cycles

    Best

  3. 12:45 AM

    Asleep by 1:00 AM

    6 h · 4 cycles

    Under 7 h

  4. 2:15 AM

    Asleep by 2:30 AM

    4 h 30 min · 3 cycles

    Under 7 h

Or choose how long to sleep

hmin

For 8 h of sleep, go to bed at:

10:45 PM

Asleep by 11:00 PM

8 h

Within 7–9 h

That wakes you 30 min into your 6th cycle, which can leave you groggy. Going to bed at 11:15 PM (7 h 30 min) or 9:45 PM (9 h) would line up with a cycle.

Bedtime reminder

A calendar alert on your phone or computer. Websites can't set your alarm clock, so this is the closest thing.

minutes

These times are estimates. They assume 15 minutes to fall asleep and 90-minute sleep cycles, but real cycles vary from about 70 to 120 minutes between people and through the night. If you often wake up groggy at these times, adjust the settings.

Adults need 7–9 hours of sleep (7–8 if you're 65 or over). This isn't medical advice: if you regularly sleep badly, talk to a doctor.

Settings

90-min cycles · 15 min to fall asleep · 12-hour clock

Nothing you type is uploaded. The times are worked out on this page, and the link in the address bar keeps them so you can bookmark or share the result. Your settings (cycle length, time to fall asleep, clock and last tab) are remembered on this device only.

Also try:Reaction Time Test

How to use the Sleep Calculator

  1. 1Pick what you know: Wake time to find bedtimes, Bedtime to find wake-up times (or press Bed now), or Hours slept to check a night that's over.
  2. 2Change the time to yours, to the minute: it starts at 7:00 AM for waking up and 11:00 PM for bed. The results update as soon as the time is complete.
  3. 3Pick one of the times marked Best: they give 7 to 9 hours of sleep, the amount adults need. Or type how many hours you want under the list.
  4. 4If you often wake up groggy at the suggested times, change the cycle length or the time to fall asleep in Settings. They're remembered on this device.
  5. 5To be reminded at bedtime, set how early and press Add to calendar, then open the file on your phone or computer.

How sleep cycles work

Through the night you cycle between non-REM sleep and REM sleep several times. Each cycle moves from light sleep (stages 1 and 2) into deep sleep (stage 3) and then into REM sleep, when most dreaming happens. The first REM period comes about 90 minutes after falling asleep, which is where the 90-minute figure comes from.

The cycles change as the night goes on. Deep sleep, the stage you need to feel refreshed, comes in longer stretches in the first half of the night. REM periods grow longer towards morning. So a cycle at 1 AM isn’t the same as one at 6 AM, and their length varies from person to person.

Why waking at the end of a cycle helps

Waking from deep sleep is hard: your heart rate and breathing are at their slowest, your muscles are relaxed, and it’s difficult to wake you. If your alarm goes off then, you’re more likely to feel groggy for a while. At the end of a cycle, sleep is lighter, so waking up tends to be easier.

That’s all the calculator does: it lines your bedtime or your alarm up with the end of a cycle. It can’t know your own cycles, so it’s a rule of thumb, not a measurement. If the suggested times leave you groggy, adjust the cycle length in Settings until they don’t.

How much sleep adults need

Age Recommended May be appropriate
18 to 25 7 to 9 hours 6 to 11 hours
26 to 64 7 to 9 hours 6 to 10 hours
65 and over 7 to 8 hours 5 to 9 hours

Source: the National Sleep Foundation’s recommendations. Lining up with cycles doesn’t make up for too little sleep: 4 cycles is 6 hours, short of what most adults need, however well the alarm is timed.

Getting the most from the times

  • Keep to a regular wake-up time, weekends included. Your body settles into a rhythm, and the bedtimes here only help if you get up around the same time.
  • Count the time to fall asleep honestly. If you usually read or scroll for half an hour, set the time to fall asleep to 30 minutes, or go to bed earlier.
  • Change the settings if the times don’t suit you. One groggy morning tells you little; a week of them is worth acting on.

If you regularly struggle to fall asleep, wake often in the night, snore loudly or feel tired however long you sleep, talk to a doctor. This calculator can’t tell you why.

Sources

Questions

How does the sleep calculator work?
Sleep comes in cycles of light, deep and REM sleep that last about 90 minutes each. Waking at the end of a cycle, when sleep is lightest, tends to feel easier than waking in the middle of deep sleep. The calculator counts whole cycles back from your wake-up time and adds 15 minutes to fall asleep. To wake at 7:00 AM, go to bed at 9:45 PM (6 cycles, 9 hours) or 11:15 PM (5 cycles, 7.5 hours).
Is the 90-minute cycle exact?
No, it's an average. Cycles run from roughly 70 to 120 minutes, differ from person to person, and change through the night, so treat the times as estimates rather than a schedule to the minute. If you wake up groggy at the suggested times, try a cycle length of 100 or 110 minutes, or a longer time to fall asleep. Cycle length can be set from 60 to 120 minutes.
How much sleep do I need?
Adults aged 18 to 64 need 7 to 9 hours a night, and adults 65 and over 7 to 8, according to the National Sleep Foundation. The American Academy of Sleep Medicine recommends at least 7. That's why only the options that give 7 to 9 hours are marked Best: with the usual 90-minute cycles, those are 5 and 6 cycles.
Can I choose a number of hours instead of cycles?
Yes. Under the list, type how long you want to sleep, from 1 minute up to 16 hours. To get 8 hours and wake at 7:00 AM, go to bed at 10:45 PM; going to bed at 11:00 PM, set your alarm for 7:15 AM. It also tells you if that amount stops partway through a cycle, and which times nearby would end on a cycle instead.
Does it work for times after midnight?
Yes. A bedtime of 12:45 AM to wake at 7:00 AM comes out correctly, midnight shows as 12:00 AM (or 00:00), and Hours slept counts across midnight: 11:00 PM to 7:00 AM is 8 hours in bed. It also works within one day, for a nap or after a night shift.
How does the bedtime reminder work?
It's a calendar file (.ics) made on this page. Open it and your phone or computer adds a Bedtime event to your calendar, every night or just once, with an alert 0 to 120 minutes before (30 by default). If tonight's alert time has already passed, it starts tomorrow. Websites can't set your phone's alarm clock, so this is the closest thing. To stop it, delete the event in your calendar.
Why did it refuse the number I typed?
Cycle length must be 60 to 120 minutes, time to fall asleep 0 to 60, the reminder 0 to 120, and the amount of sleep you choose 1 minute to 16 hours, in whole minutes. Anything outside is refused with a message saying so, rather than quietly changed into something you didn't type.
Is anything I type saved or sent anywhere?
Nothing is uploaded. The times are worked out on this page, so it keeps working if you go offline. The link in the address bar holds your times after the # so you can bookmark or share a result, and browsers never send that part to a server. Your settings (cycle length, time to fall asleep, clock and last tab) are remembered in this browser on this device only; the times you type aren't. Forget my settings clears them.
Can I use it for a child?
It's made for adults. Babies' sleep cycles are much shorter, around 50 to 60 minutes, and children need more sleep than adults, so the times and the 7 to 9 hour guide wouldn't fit. Ask your child's doctor about how much sleep they need.