Sleep Calculator
Enter the time you need to wake up and see bedtimes that line up with full sleep cycles, or enter when you will fall asleep to see wake-up times.
Last updated November 2, 2026
Result
Go to bed around 9:45 PM for 6 full cycles
| Bedtime | Sleep cycles | Time asleep |
|---|---|---|
| 9:45 PM | 6 | 9 hours |
| 11:15 PM | 5 | 7.5 hours |
| 12:45 AM | 4 | 6 hours |
Assumes about 15 minutes to fall asleep and 90-minute sleep cycles, which vary by person and night. Most adults do well with 7 or more hours; needs differ. Ongoing sleep problems are worth discussing with a health professional. General estimate only, not medical advice. Talk to a qualified health professional about your own situation.
Show the work
Formula: Bedtime = wake-up time − cycles × 90 minutes − 15 minutes to fall asleep.
- You entered 7:00 AM as your wake-up time.
- 6 cycles × 90 = 540 minutes (9 hours): 7:00 AM − 540 − 15 minutes = 9:45 PM.
- 5 cycles × 90 = 450 minutes (7.5 hours): 7:00 AM − 450 − 15 minutes = 11:15 PM.
- 4 cycles × 90 = 360 minutes (6 hours): 7:00 AM − 360 − 15 minutes = 12:45 AM.
Result: Go to bed around 9:45 PM for 6 full cycles
How the times are worked out
bedtime = wake-up time − (cycles × 90 minutes) − 15 minutes to fall asleep
Sleep moves through repeating stages in cycles that average about 90 minutes, and waking at the end of a cycle is often easier than waking in the middle of one. The calculator counts back 4, 5 or 6 cycles (6, 7.5 or 9 hours) from your wake-up time and adds 15 minutes for falling asleep. Going the other way, it adds the same time to your sleep start.
To wake at 7:00 AM, the bedtimes are 9:45 PM (6 cycles), 11:15 PM (5 cycles) and 12:45 AM (4 cycles). Most adults are advised to sleep at least 7 hours, so the 5 and 6 cycle options are the ones to aim for.
See how we calculate for the formulas, sources and review schedule behind every calculator.
This is an estimate, not medical advice
Sleep needs differ by age and person, and the 90-minute cycle is an average. This tool is a planning aid, not a sleep assessment. If you often feel unrested or have ongoing trouble sleeping, talk to a doctor. This page has not been reviewed by a medical professional.
Sources
Official and primary sources for the statements on this page. Their authors have not reviewed or endorsed this page.
- Hirshkowitz M, Whiton K, Albert SM, et al. National Sleep Foundation’s sleep time duration recommendations: methodology and results summary. Sleep Health 2015;1(1):40-43. Recommends 7 to 9 hours of sleep for young adults and adults and 7 to 8 hours for older adults. The 5-cycle (7.5 hours) and 6-cycle (9 hours) options fall within that range; the 4-cycle option (6 hours) does not
- Institute of Medicine, Sleep Disorders and Sleep Deprivation: An Unmet Public Health Problem, chapter 2 "Sleep Physiology" (National Academies Press, 2006). Gives the first NREM-REM cycle as 70 to 100 minutes and later cycles as about 90 to 120 minutes, so the fixed 90-minute cycle used here is an approximation
The 15 minutes allowed to fall asleep is an assumption without a source. Cycle length and time to fall asleep differ between people and from night to night.
How to use the Sleep Calculator
- Choose whether you know your wake-up time or the time you will fall asleep.
- Enter the time.
- Read the suggested times, then use Copy, Print or Share.
Worked example
To wake at 7:00 AM, count back full cycles plus 15 minutes to fall asleep: 6 cycles = 9:45 PM, 5 cycles = 11:15 PM, 4 cycles = 12:45 AM.
| Bedtime | Sleep cycles | Time asleep |
|---|---|---|
| 9:45 PM | 6 | 9 hours |
| 11:15 PM | 5 | 7.5 hours |
| 12:45 AM | 4 | 6 hours |
Frequently asked questions
How long is a sleep cycle?
About 90 minutes on average, though it varies between people and through the night.
How much sleep do I need?
Most adults are advised to get at least 7 hours. Teens and children need more, and needs vary.
Why add 15 minutes?
It is a typical time to fall asleep. If you fall asleep faster or slower, shift the times.
Will waking at the end of a cycle always feel better?
Not always. It is a rule of thumb, and total sleep time and sleep quality matter more.
Are my numbers stored or sent anywhere?
No. The calculation runs in your browser and nothing you enter is saved or uploaded.