How to use the sleep calculator
- Choose a mode: I want to wake up at to find bedtimes, or I'm going to bed at to find wake-up times. The bedtime starts at the current time on your device.
- Enter the time.
- Adjust the time to fall asleep if needed. The default is 15 minutes; you can enter anything from 0 to 60.
- Press Calculate. You get three options with the hours of sleep and the number of cycles for each. The options that give 7 to 9 hours of sleep are marked as recommended.
How it works
During the night you go through repeating sleep cycles of light sleep, deep sleep and REM sleep. One cycle lasts roughly 90 minutes. Waking up at the end of a cycle, when sleep is light, usually feels easier than waking up from deep sleep.
- Bedtime = wake-up time − time to fall asleep − cycles × 90 minutes
- Wake-up time = bedtime + time to fall asleep + cycles × 90 minutes
The calculator shows 6, 5 and 4 cycles, which equal 9, 7.5 and 6 hours of actual sleep. Times that cross midnight are labeled as the day before or the next day.
Worked example
You want to wake up at 7:00 AM and usually fall asleep in 15 minutes:
- 6 cycles: 7:00 AM − 15 min − 9 h = 9:45 PM
- 5 cycles: 7:00 AM − 15 min − 7.5 h = 11:15 PM
- 4 cycles: 7:00 AM − 15 min − 6 h = 12:45 AM
If you go to bed at 11:00 PM instead, the wake-up times are 5:15 AM (4 cycles), 6:45 AM (5 cycles) and 8:15 AM (6 cycles).
Good to know
- Sleep experts recommend 7 to 9 hours a night for most adults. Teenagers and children need more.
- The 90-minute cycle is an average. Real cycles vary between about 70 and 120 minutes and change during the night, so treat the times as a guide.
- A regular schedule, even at weekends, helps more than any single perfect bedtime.
- If you often feel tired after enough sleep, snore loudly or have trouble falling asleep, talk to a doctor.
- Results are estimates, not medical advice.