Sleep Calculator — best bedtime for your wake-up time

How it works

During the night your brain moves through repeating cycles of non-REM and REM sleep. According to the NHLBI, a cycle "starts over every 80 to 100 minutes" — commonly rounded to 90 minutes — and there are "usually four to six cycles per night". You may wake briefly between cycles, which is why finishing a whole cycle tends to feel easier than being roused from deep sleep partway through.

The calculator uses this to work backward. Starting from your wake-up time it subtracts whole 90-minute cycles:

• 6 cycles ≈ 9 hours of sleep
• 5 cycles ≈ 7.5 hours of sleep
• 4 cycles ≈ 6 hours of sleep

Pick whichever bedtime gives you enough rest and is realistic for tonight. Because it counts in whole cycles, your alarm is more likely to land in lighter sleep.

These are guides, not guarantees. Real cycle length varies from night to night and person to person, and it also takes time to actually fall asleep — so give yourself a little margin before the bedtime shown. What matters most is getting enough total sleep on a consistent schedule.

Worked example

To wake at 7:00 AM, count back in 90-minute cycles. Six cycles is 9 hours, so a 10:00 PM bedtime; five cycles is 7.5 hours, giving 11:30 PM; four cycles is 6 hours, giving 1:00 AM. Being asleep by 11:30 PM lines your 7:00 AM alarm up with the end of a cycle.

Frequently asked questions

What time should I go to bed to wake up at 7 AM?

Count back whole 90-minute sleep cycles. For a 7:00 AM wake-up, that is about 10:00 PM for six cycles (9 hours), 11:30 PM for five cycles (7.5 hours) or 1:00 AM for four cycles (6 hours). Allow a little extra time to fall asleep.

How long is a sleep cycle?

The NHLBI says a full non-REM/REM cycle starts over every 80 to 100 minutes, which is commonly rounded to 90 minutes. A typical night contains four to six of these cycles.

Why count sleep in 90-minute cycles?

You briefly surface near the end of each cycle, so waking then usually feels easier than being pulled out of deep sleep mid-cycle. Timing your alarm to the end of a whole cycle is the idea behind cycle-based bedtimes.

How many sleep cycles do I need?

Most nights run four to six cycles. Five cycles is about 7.5 hours and six is about 9 hours; four (6 hours) is on the short side for many adults. Choose the option that lets you get enough total rest.

Does this account for the time it takes to fall asleep?

No — the times shown are when you should be *asleep*. Since falling asleep usually takes a little while, set your actual bedtime somewhat earlier than the time listed.

Are 90-minute cycles exact?

No. Ninety minutes is an average; real cycles vary between roughly 80 and 100 minutes and change through the night and between people. Treat the bedtimes as a helpful guide rather than a precise rule.