Sleep Calculator

Sleep is not one long, even state. It runs in cycles, and each cycle goes from light sleep to deep sleep and to dream sleep, and then starts again. If your alarm rings at the end of a cycle, when your sleep is light, you often wake up more easily than if it rings in the middle of deep sleep. The Sleep Calculator counts back from your wake-up time in whole cycles, to find the times to go to bed.

What you enter

• What you know: the time you want to wake up, or the time you are getting into bed.
• The time: your wake-up time or your bedtime.
• Minutes to fall asleep: how long it takes you to fall asleep after you get into bed. About 15 minutes is usual.
• The length of a sleep cycle: about 90 minutes for most people. It can be 80 to 110 minutes for you.
• The clock style: 12-hour or 24-hour.

How the results are worked out

A number of cycles is chosen, and the time is counted back or forward from the time you entered:

to wake at a set time:
  bedtime = wake-up time - (cycles x cycle length) - time to fall asleep

to go to bed at a set time:
  wake-up time = bedtime + time to fall asleep + (cycles x cycle length)

The calculator shows 6, 5, 4 and 3 cycles. Times are counted in minutes around the 24-hour clock, so the result does not depend on the date, or on a change of the clocks.

A worked example

You want to wake up at 7:00 AM. You take 15 minutes to fall asleep, and a cycle is 90 minutes.

• Cycles: 6
  Sleep: 9 hours
  Calculation: 7:00 AM - 9 hours - 15 minutes
  Get into bed at: 9:45 PM
• Cycles: 5
  Sleep: 7 hours 30 minutes
  Calculation: 7:00 AM - 7 hours 30 minutes - 15 minutes
  Get into bed at: 11:15 PM
• Cycles: 4
  Sleep: 6 hours
  Calculation: 7:00 AM - 6 hours - 15 minutes
  Get into bed at: 12:45 AM
• Cycles: 3
  Sleep: 4 hours 30 minutes
  Calculation: 7:00 AM - 4 hours 30 minutes - 15 minutes
  Get into bed at: 2:15 AM

The times are for getting into bed, and not for falling asleep. The 15 minutes that you need to fall asleep are already counted. If you want to be asleep at 11:15 PM, you should be in bed at 11:00 PM.

Chart: one night

The night with 5 cycles. Each block is 15 minutes. Grey is the time to fall asleep, and green is the five sleep cycles.

███████████████████████████████
11:15 PM  bed                              7:00 AM  wake

Chart: how many cycles?

Green is a good night for most adults, and amber is short.

3 cycles  ████████████████████ 4.5 hours
4 cycles  ███████████████████████████ 6 hours
5 cycles  █████████████████████████████████ 7.5 hours
6 cycles  ████████████████████████████████████████ 9 hours

Different wake-up times

The times to get into bed, with a cycle of 90 minutes and 15 minutes to fall asleep:

You wake up at	6 cycles (9 hours)	5 cycles (7.5 hours)
5:00 AM	7:45 PM	9:15 PM
6:00 AM	8:45 PM	10:15 PM
7:00 AM	9:45 PM	11:15 PM
8:00 AM	10:45 PM	12:15 AM

A quick rule: for 5 cycles, go to bed 7 hours 45 minutes before you wake up. For 6 cycles, go to bed 9 hours 15 minutes before.

Your own numbers

The numbers of 90 and 15 are averages. Try your own. For a wake-up time of 7:00 AM and 5 cycles:

Change	Get into bed at
Fall asleep at once (0 minutes)	11:30 PM
15 minutes to fall asleep (the example)	11:15 PM
30 minutes to fall asleep	11:00 PM
A cycle of 80 minutes	12:05 AM
A cycle of 100 minutes	10:25 PM

If you often wake up in the middle of the night, or feel tired after 7.5 hours, your cycles may be shorter or longer, or you may need more sleep. Try a different length for a few days.

What happens in a cycle

• Light sleep. You drift off, and your heart rate and breathing slow down. It is easy to wake.
• Deep sleep. The body repairs itself, and it is hard to wake you. Most of it is in the first half of the night.
• REM, or dream sleep. The brain is active, and most dreams happen. REM periods get longer towards the morning.

Waking up in deep sleep can leave you feeling groggy, which is called sleep inertia. Waking in light sleep, or at the end of a cycle, is usually easier.

How much sleep do people need?

Sleep organizations give ranges by age. Needs differ from person to person, so these are a general guide. The green bar is the lowest amount in the range, and the amber part is the rest of the range:

Newborn (0-3 months)  ██████████████████████████████████ 14-17 hours
Infant (4-11 months)  ██████████████████████████████ 12-15 hours
Toddler (1-2 years)   ████████████████████████████ 11-14 hours
Preschool (3-5)       ██████████████████████████ 10-13 hours
School age (6-13)     ██████████████████████ 9-11 hours
Teenager (14-17)      ████████████████████ 8-10 hours
Adult (18-64)         ██████████████████ 7-9 hours
Older adult (65+)     ████████████████ 7-8 hours

For children and teenagers, the calculator works in the same way, but you should use the number of cycles that gives them the hours they need. For example, 6 cycles (9 hours) is a good choice for a teenager.

Tips for better sleep

• Keep a regular schedule. Go to bed and wake up at about the same times every day, even on weekends.
• Get natural light in the morning, and keep the evening dim.
• Avoid caffeine in the afternoon, and a heavy meal or alcohol close to bedtime.
• Keep the bedroom dark, quiet and cool.
• Put screens away for a while before bed.
• Do not lie in bed awake for a long time. If you cannot sleep, get up and do something quiet until you feel sleepy.

What this calculator does not include

• Your own sleep pattern. A cycle is a natural rhythm that changes with age, and from night to night, so 90 minutes is only an average.
• Sleep disorders such as sleep apnea or insomnia. If you are always tired, or have trouble sleeping, talk to a doctor.
• Naps. A short nap of 20 minutes, or a full cycle of 90 minutes, is different from a night of sleep.

This tool gives a general guide only. It is not medical advice. If you have problems with your sleep, see a doctor.