Sleep calculator
Choose when you want to wake up, or when you are going to bed. You get times that fit whole sleep cycles of about 90 minutes, with about 15 minutes to fall asleep.
Updated 21 October 2026. Sources named on the page.
Key factMost adults need 7 to 9 hours of sleep (Sleep Health Foundation, Australia), which is 5 or 6 cycles of about 90 minutes; to wake at 7:00 am with 15 minutes to fall asleep, go to bed at 11:15 pm or 9:45 pm.
- Wake time to bedtime, or bedtime to wake time
- Whole 90 minute cycles plus about 15 minutes to fall asleep
- Shows 4, 5 and 6 cycles (6, 7.5 and 9 hours)
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The cycles and the times
Bedtime = wake time minus the cycles (90 minutes each) minus about 15 minutes to fall asleep. Waking at the end of a cycle is often easier than waking in the middle of one, but the lengths are averages and vary between people and nights.
Cycles and hours
| Cycles | Hours of sleep |
|---|---|
| 4 | 6 |
| 5 | 7.5 |
| 6 | 9 |
Worked examples
| Wake time | 6 cycles | 5 cycles | 4 cycles |
|---|---|---|---|
| 7:00 am | 9:45 pm | 11:15 pm | 12:45 am |
| 6:00 am | 8:45 pm | 10:15 pm | 11:45 pm |
Sources: The 90 minute cycle and 15 minutes to fall asleep are commonly used rules of thumb, not measurements of any person; Sleep Health Foundation (Australia): adults generally need 7 to 9 hours of sleep
General information, not medical advice.
How to use the calculator
Choose whether you know your wake time or your bedtime, enter the time and press the button. The calculator lists times that fit whole sleep cycles of about 90 minutes, and it adds about 15 minutes for falling asleep.
For a wake time of 7:00 am it suggests bedtimes such as 9:45 pm for six cycles, 11:15 pm for five and 12:45 am for four.
Reading the result
More cycles means more sleep: five cycles is 7.5 hours and six is 9 hours. The calculator does not say how many you need, because that differs from person to person. The sleep guide explains the common ranges and what to do if you are not sure.
If you often cannot sleep or feel very tired despite enough hours, talk to a doctor. A sleep diary printable makes a good starting point for that conversation.
Questions
How many hours of sleep do adults need?
Most adults need about 7 to 9 hours a night. Needs differ between people, so how you feel in the day matters too.
What is a sleep cycle?
Sleep moves through lighter and deeper stages in repeating cycles. A cycle is often taken as about 90 minutes, but it varies from person to person and night to night.
Why add 15 minutes?
It allows for the time many people take to fall asleep. If you fall asleep faster or slower, shift the times to suit.
Does waking at the end of a cycle guarantee I feel rested?
No. It is a rule of thumb. Total sleep, regular times and your own body matter more than an exact minute.
When should I see a doctor?
If you are often tired, snore loudly, stop breathing in your sleep or sleep badly for weeks, talk to a doctor. This tool is not medical advice.
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