How the bedtime calculator works
Sleep isn’t one long, even stretch. Through the night you move through repeating cycles of light sleep, deep sleep and REM (dreaming) sleep. One full cycle lasts about 90 minutes on average.
Waking from deep sleep is what often leaves you groggy. Waking near the end of a cycle, when sleep is lighter, tends to feel easier. So the calculator works backwards from your alarm:
- Start with your wake-up time.
- Subtract whole 90-minute cycles: 6 cycles (9 hours), 5 cycles (7.5 hours) or 4 cycles (6 hours).
- Subtract your fall-asleep allowance, because your first cycle only begins once you’re asleep.
Every bedtime is shown in both 12-hour and 24-hour time, so you can copy it straight into any clock or phone alarm.
Worked example: waking at 6:00 AM
Say your alarm is set for 6:00 AM and you usually take 15 minutes to drift off. You need to be asleep by 6:00 AM minus the sleep you want, and in bed 15 minutes before that.
| Sleep | Asleep by | Go to bed | 24-hour |
|---|---|---|---|
| 9 h · 6 cycles | 9:00 PM | 8:45 PM | 20:45 |
| 7.5 h · 5 cycles | 10:30 PM | 10:15 PM | 22:15 |
| 6 h · 4 cycles | 12:00 AM | 11:45 PM | 23:45 |
If you take 30 minutes to fall asleep instead, each bedtime moves 15 minutes earlier: 8:30 PM, 10:00 PM and 11:30 PM.
Choosing the right bedtime
Health guidance recommends that adults get at least 7 hours of sleep a night on a regular basis. That’s why the 9-hour and 7.5-hour options are marked recommended. Teenagers and children need more.
A consistent schedule matters as much as the exact time. Going to bed and waking at roughly the same time each day, including weekends, helps your body clock settle. If you wake up before your alarm feeling rested, that’s a good sign the timing suits you.
When to seek help
Talk to a doctor or a sleep specialist if you:
- regularly take longer than 30 minutes to fall asleep, or wake in the night and can’t get back to sleep;
- snore loudly, or someone notices you gasp or stop breathing while asleep;
- feel very sleepy during the day, especially while driving or working, despite enough time in bed;
- have had trouble sleeping for several weeks and it’s affecting your mood, health or daily life.
Frequently asked questions
What time should I go to bed if I wake up at 6 AM?
Allowing 15 minutes to fall asleep, aim for 8:45 PM (20:45) for 9 hours of sleep or 10:15 PM (22:15) for 7.5 hours. 11:45 PM (23:45) gives 6 hours, which is fine now and then but short for most adults.
What if I miss my bedtime?
Go to bed when you are sleepy rather than staying up to hit the next cycle. The next option on the list is 90 minutes later, so it’s usually better to take the extra sleep than chase a perfect finish.
Are sleep cycles always 90 minutes?
No. 90 minutes is an average. Cycles typically run somewhere around 80 to 120 minutes, and the balance of deep and REM sleep shifts as the night goes on. Use the times as a starting point, then adjust to how you feel.
How should I set the fall-asleep allowance?
Think about a typical night: from lights out to asleep. Choose anywhere from 5 to 30 minutes. If you regularly need longer than 30 minutes, see the note on when to seek help below.
Does this work for shift workers?
Yes. The maths works for any wake-up time, day or night. Choose a PM wake-up time and the calculator will count back into the morning or afternoon.
Related calculators
Wake-up time calculator
Going to bed now or at a set time? Find wake-up times that land at the end of a cycle.
Nap calculator
Plan a 10-, 20- or 90-minute nap and see when to set your alarm.
Sleep needs by age
See the recommended hours of sleep for any age, from newborns to older adults.
Sleep debt calculator
Add up the last seven nights and see how far you are from your sleep target.
Sources
- National Institute of Neurological Disorders and Stroke. Brain Basics: Understanding Sleep.
- Centers for Disease Control and Prevention. About Sleep.
- Watson NF, et al. Recommended Amount of Sleep for a Healthy Adult: A Joint Consensus Statement of the American Academy of Sleep Medicine and Sleep Research Society. Journal of Clinical Sleep Medicine, 2015;11(6):591–592. doi:10.5664/jcsm.4758.
- National Heart, Lung, and Blood Institute. How Sleep Works.