Sleeping in cycles
During the night, sleep goes through repeating cycles of about 90 minutes: light sleep, deep sleep and REM sleep, when most dreaming happens. Waking up in the middle of deep sleep often leaves you groggy, while waking near the end of a cycle usually feels easier. This calculator counts back whole cycles from your wake-up time to suggest bedtimes, or counts forward from your bedtime to suggest wake-up times.
The formulas
- Bedtime = wake-up time − cycles × cycle length − time to fall asleep
- Wake-up time = bedtime + time to fall asleep + cycles × cycle length
Worked example
You need to get up at 7:00 AM and usually take 15 minutes to fall asleep:
- 5 cycles of 90 minutes = 7 h 30 min of sleep.
- 7:00 AM − 7 h 30 min − 15 min = 11:15 PM.
- For 6 cycles (9 hours of sleep): 9:45 PM.
- For 4 cycles (6 hours), 12:45 AM, a short night.
How much sleep do you need?
| Age | Recommended sleep per 24 hours |
|---|---|
| 1 to 2 years | 11 to 14 hours, naps included |
| 3 to 5 years | 10 to 13 hours, naps included |
| 6 to 12 years | 9 to 12 hours |
| 13 to 18 years | 8 to 10 hours |
| Adults | 7 hours or more |
These ranges come from the American Academy of Sleep Medicine. Five or six cycles fit adult needs; four cycles fall short if repeated night after night.
Limits of the 90-minute rule
Cycle length varies from about 70 to 120 minutes between people, and cycles change through the night: deep sleep dominates early, REM sleep later. Use the suggested times as a starting point. If you wake up tired, shift your bedtime by 15 minutes and try for a week.
Better sleep habits
- Keep a regular schedule, including weekends.
- Get daylight in the morning and dim screens and lights in the hour before bed.
- Avoid caffeine in the afternoon and large meals or alcohol close to bedtime.
- Keep the bedroom cool, dark and quiet.
Persistent trouble falling or staying asleep, loud snoring or daytime sleepiness despite enough time in bed deserve a talk with a doctor.