How to use the sleep calculator
- Choose what you know. Pick Wake-up time if you have to be up at a set time and want to know when to go to bed. Pick Bedtime if you are going to bed now, or at a set time, and want to know when to set the alarm. In Bedtime mode the time starts at the current time, and the Now button sets it again.
- Choose your age group. The times that give the hours of sleep the CDC recommends for that age are marked with a dot.
- Adjust the settings if you like. Under Sleep cycle settings you can change how long you usually take to fall asleep (15 minutes to start with) and the length of one sleep cycle (90 minutes to start with).
The big time is the latest bedtime, or the earliest alarm, that still gives the recommended hours for your age. The list shows every option from 3 cycles up. Copy times gives you the list as text.
Why it counts in sleep cycles
Sleep is not one long, even state. Through the night you move through lighter and deeper stages of non-REM sleep and then REM sleep, and then the pattern begins again. The US National Heart, Lung, and Blood Institute describes it like this:
"The cycle starts over every 80 to 100 minutes. Usually there are four to six cycles per night."National Heart, Lung, and Blood Institute, How Sleep Works
About 90 minutes, the middle of that range, is the length commonly used by sleep calculators, and it is what this one starts with. The idea is to plan the alarm for the end of a cycle rather than the middle of one. Real cycles vary from person to person and through the night, so treat the times as a guide, not a promise. If you know your own pattern, change the cycle length in the settings.
How the times are worked out
Bedtime = wake-up time − time to fall asleep − (cycles × cycle length)
Wake-up time = bedtime + time to fall asleep + (cycles × cycle length)
With a 7:00 AM alarm, 15 minutes to fall asleep and 90-minute cycles:
| Cycles | Sleep | Go to bed at |
|---|---|---|
| 6 | 9 hours | 9:45 PM |
| 5 | 7.5 hours | 11:15 PM |
| 4 | 6 hours | 12:45 AM |
| 3 | 4.5 hours | 2:15 AM |
For an adult aged 18 to 60, 5 cycles (7.5 hours) is the shortest option that meets the CDC's "7 or more hours", so the calculator shows 11:15 PM. Going the other way, if you go to bed at 10:30 PM, 5 cycles mean an alarm at 6:15 AM and 6 cycles at 7:45 AM.
Times wrap around midnight. A 1:00 AM wake-up with 5 cycles means going to bed at 5:15 PM the day before, and a bedtime of 12:30 AM gives a 5-cycle alarm at 8:15 AM. In the US the times show in 12-hour format; with a UK English browser they show in 24-hour format.
How much sleep the CDC recommends
The US Centers for Disease Control and Prevention lists these amounts of sleep per day by age:
| Age group | Age | Recommended |
|---|---|---|
| Newborn | 0 to 3 months | 14 to 17 hours |
| Infant | 4 to 12 months | 12 to 16 hours, including naps |
| Toddler | 1 to 2 years | 11 to 14 hours, including naps |
| Preschool | 3 to 5 years | 10 to 13 hours, including naps |
| School age | 6 to 12 years | 9 to 12 hours |
| Teen | 13 to 17 years | 8 to 10 hours |
| Adult | 18 to 60 years | 7 or more hours |
| Adult | 61 to 64 years | 7 to 9 hours |
| Adult | 65 years and older | 7 to 8 hours |
The calculator offers the age groups from 6 years up. Younger children's totals include naps, so a single bedtime and alarm do not describe their day well.
What this calculator is and is not
This is a planning aid that does simple clock arithmetic. It is general information, not medical advice, and it cannot tell you whether your sleep is healthy. Many things affect sleep, from light and noise to caffeine, shift work and health conditions. The CDC's advice is plain:
"Talk to your healthcare provider if you have problems sleeping."CDC, About Sleep
If you often have trouble falling or staying asleep, or you feel very sleepy during the day, talk to a doctor.
Questions and answers
What time should I go to bed to wake up at 7 AM?
With 15 minutes to fall asleep and 90-minute cycles, 11:15 PM gives 5 cycles (7.5 hours of sleep) and 9:45 PM gives 6 cycles (9 hours). For adults aged 18 to 60 the CDC recommends 7 or more hours.
How long is a sleep cycle?
The National Heart, Lung, and Blood Institute says the cycle starts over every 80 to 100 minutes, with usually four to six cycles a night. 90 minutes is the length commonly used for planning, and you can change it in the settings.
Why add time to fall asleep?
Most people do not fall asleep the moment they lie down. The calculator adds 15 minutes to start with; set it to what is usual for you.
Is it better to sleep 6 hours or 7.5 hours?
For adults, the CDC recommends 7 or more hours, so 6 hours falls short even if it ends at the end of a cycle. The calculator marks the options that meet the recommended hours for your age.
Can a sleep calculator fix my sleep problems?
No. It only works out times. If you often have trouble sleeping or feel tired during the day, talk to a doctor or other healthcare provider.
Sources
Checked on 7 October 2026. If a rule has changed, please tell us.