How the sleep calculator works
A night’s sleep is a series of cycles of about 90 minutes. The calculator counts whole cycles back from the time you need to wake up (or forward from your bedtime), and adds the time it takes you to fall asleep, so your alarm rings when sleep is light.
Example
To wake at 7:00 am after five cycles (7½ hours of sleep), and taking 15 minutes to fall asleep, go to bed at 11:15 pm: 7:00 am − 7 h 30 min − 15 min.
Tips for better sleep
- Keep the same wake-up time every day, weekends included. It sets your body clock more than bedtime does.
- Get daylight in the morning and dim the lights in the evening.
- Stop caffeine about eight hours before bed, and keep the bedroom cool, dark and quiet.
Frequently asked questions
Why does the sleep calculator use 90-minute cycles?
Sleep runs in cycles of light, deep and dream (REM) sleep that last about 90 minutes each. Waking at the end of a cycle, when sleep is light, usually feels easier than being woken in the middle of deep sleep, even if the total time is a little shorter.
How many sleep cycles do I need?
Most adults need 7 to 9 hours, which is about five or six cycles. Teenagers need 8 to 10 hours. Four cycles (6 hours) is fine now and then, but not every night.
What does “time to fall asleep” mean?
Most people take 10 to 20 minutes to drift off. The calculator adds this to your bedtime so the cycles start when you're actually asleep. Change it if you usually fall asleep faster or slower.
Is this medical advice?
No. It's a planning aid based on average sleep cycles. If you're often tired however long you sleep, or you snore loudly or wake up gasping, talk to a doctor.