Sleep calculator

Enter the time you need to wake up, and this sleep calculator shows what time to go to bed so you wake at the end of a sleep cycle and feel refreshed. You can also see the best times to wake up if you go to bed now.

What time do you want to wake up?

Time to fall asleep

Go to bed at

9:45 PMRecommended
9 hr of sleep · 6 cycles
11:15 PMRecommended
7 hr 30 min of sleep · 5 cycles
12:45 AM
6 hr of sleep · 4 cycles

Based on 90-minute sleep cycles plus 15 minutes to fall asleep. “Recommended” marks 5–6 cycles, which fall within the 7–9 hours advised for adults.

How it works

Through the night, sleep moves from light sleep (non-REM stages 1 and 2) down into deep sleep (stage 3) and back up into REM sleep, over and over. The first REM period comes about 90 minutes after you fall asleep, and one cycle usually lasts around 90 minutes.

This calculator counts back six, five, and four 90-minute cycles from your wake-up time, then subtracts the time it takes you to fall asleep to give you a bedtime. Choose “By bedtime,” and it works the other way around: it adds the time it takes to fall asleep and the cycles to your bedtime, then shows good times to wake up.

Why waking at the end of a cycle feels easier

That groggy, heavy feeling you have for a while after waking up is called sleep inertia. Research suggests it’s stronger and lasts longer when you wake in the middle of deep sleep. The point of this calculator is to time your alarm for the end of a cycle, when your sleep has gotten lighter.

How much sleep do you need?

The National Sleep Foundation (NSF) recommends 7–9 hours a day for adults aged 18–64 and 8–10 hours for teens aged 14–17. The American Academy of Sleep Medicine (AASM) also recommends that adults get 7 or more hours of sleep. In cycles, that’s 5–6 cycles (7 hours 30 minutes to 9 hours) for adults, so those two rows are marked “Recommended” in the results.

The 4-cycle option (6 hours) is shown for comparison. Waking at the end of a cycle may feel easier, but it doesn’t make up for not getting enough sleep.

Why the math isn’t exact

Cycle length varies from person to person, and even from night to night for the same person. It ranges from roughly 70 to 120 minutes.
Deep sleep is longer early in the night, and REM sleep gets longer toward morning, so each cycle is made up differently.
How long it takes to fall asleep also depends on how you feel that day.

So treat the result as a starting point, not an exact answer. Pay attention to how you feel for a few days, then take the bedtime from a morning you woke up refreshed and try moving it earlier or later in 15-minute steps.

How to stick to your bedtime

Lock in your wake-up time first. If you don’t push it much later on weekends, your bedtime will settle into place, too.
Set a reminder for bedtime. Haruda Alarm can remind you to log your sleep at a time you choose, and before bed it shows how many hours you’ll sleep if you fall asleep now.
Log when you fall asleep and wake up. Just a week of logs is enough to show how much sleep works for you.

For more on how sleep cycles work, read our guide to sleep cycles.

Common questions

What should I choose for “Time to fall asleep”?

If you’re not sure, leave it at the default of 15 minutes. Lower it if you fall asleep as soon as you lie down, and raise it if you tend to toss and turn for a while.

Is 6 hours (4 cycles) of sleep enough?

Waking at the end of a cycle may feel easier, but you’re still short on sleep. Adults are advised to get at least 7 hours.

I woke up at the calculated time and still feel tired.

Cycle length and the time it takes to fall asleep vary from person to person. Try moving your bedtime 15 minutes at a time and keep a log for a few days. If you’re still sleepy during the day even though you get enough sleep, it’s a good idea to talk to a doctor or sleep specialist.