Quick answer: To wake up at 6:30 am after five 90-minute cycles (7.5 hours) and 15 minutes to fall asleep, go to bed at 10:45 pm; for six cycles (9 hours), at 9:15 pm.
How it's calculated
During the night you pass through cycles of light sleep, deep sleep and REM (dreaming) sleep, each lasting about 90 minutes on average. Waking at the end of a cycle, when sleep is lighter, tends to feel easier than waking from deep sleep.
Bedtime = wake-up time − time to fall asleep − number of cycles × 90 minutes. Going the other way, wake-up time = bedtime + time to fall asleep + cycles × 90 minutes. Most people take 10 to 20 minutes to fall asleep; change it if you know yours.
Cycle length varies between people and through the night (often 70 to 120 minutes), so treat the times as a guide. What matters most is getting enough sleep: at least 7 hours for adults, which is 5 cycles or more.
Worked examples
- Wake at 6:30 am, 15 min to fall asleep, 5 cycles: 06:30 − 15 min − 450 min = 22:45
- Bed at 10:30 pm, 15 min, 5 cycles: 22:30 + 15 min + 450 min = 06:15
- Wake at 5:00 am (Fajr), 6 cycles: 05:00 − 15 min − 540 min = 19:45
Frequently asked questions
How many hours of sleep do adults need?
At least 7 hours a night for adults aged 18 to 60, according to the CDC. Teenagers need 8 to 10 hours and school-age children 9 to 12.
Is it better to sleep 6 hours than 7?
No. Six hours ends on a full cycle, but adults who regularly sleep less than 7 hours have more health problems. Sleep five or six full cycles when you can.
Does a short afternoon nap count?
A nap of 20 to 30 minutes can refresh you without deep sleep, so you wake easily. It does not replace a full night's sleep.
How long does it normally take to fall asleep?
Usually about 10 to 20 minutes. You can change the time to fall asleep on the form to match your own habit.
Does waking at the same time help?
Yes. A steady wake-up time, even at weekends, helps set your body clock and makes it easier to fall asleep at night.
Are sleep cycles always 90 minutes?
No. 90 minutes is an average, and cycles often range from about 70 to 120 minutes. Treat the suggested times as a starting point and adjust.