Sleep Calculator

How the sleep calculator works

Sleep moves through repeating cycles of lighter and deeper sleep, including dream (REM) sleep. One cycle lasts about 90 minutes on average. Waking at the end of a cycle, when sleep is lightest, usually feels easier than being pulled out of deep sleep.

The calculator counts back (or forward) in whole cycles and allows time to fall asleep:

Bedtime = Wake-up time − (Number of cycles × 90 minutes) − Time to fall asleep

Wake-up time = Bedtime + Time to fall asleep + (Number of cycles × 90 minutes)

Worked example

You need to wake up at 7:00 AM and take about 15 minutes to fall asleep.

• 6 cycles (9 hours): 7:00 AM − 9 h − 15 min = 9:45 PM
• 5 cycles (7 h 30 min): 7:00 AM − 7 h 30 min − 15 min = 11:15 PM
• 4 cycles (6 hours): 7:00 AM − 6 h − 15 min = 12:45 AM
• 3 cycles (4 h 30 min): 7:00 AM − 4 h 30 min − 15 min = 2:15 AM

Five or six cycles fit the 7 to 9 hours that most adults need, so 9:45 PM or 11:15 PM would be your best bedtimes.

How much sleep do you need?

Adults should aim for 7 or more hours of sleep a night on a regular basis. Teenagers need more, about 8 to 10 hours, and school-age children need 9 to 12 hours. Counting cycles is a useful guide, but total sleep matters more than hitting an exact minute.

Limits of the 90-minute rule

The 90-minute figure is an average. Real cycles vary from about 70 to 120 minutes, between people and from night to night, so treat the times as a good starting point and adjust to how you feel. Sleep that is broken by noise, alcohol, caffeine or a sleep disorder will not follow the pattern. If you are often exhausted despite enough hours in bed, speak to a doctor.

How to use the sleep calculator

1. Choose what you want to find. Pick When to go to bed if you know when you must wake up, or When to wake up if you know when you are going to bed.
2. Enter the time. Type it as 7:00 AM, 7am, 07:00 or 19:30.
3. Set how long you take to fall asleep. The default is 15 minutes. Increase it if you usually lie awake for longer.
4. Read the results. The best time is based on five cycles (7.5 hours), with a six-cycle alternative. The table shows every option from three to six cycles.
5. Change the cycle length (optional). Open Advanced options if you know your cycles run shorter or longer than 90 minutes.

Aim to be in bed at the earlier time, not just to start winding down then. Keeping the same sleep and wake times every day, including weekends, makes the pattern work better.