Sleep Calculator guide
What time should I go to bed? Bedtimes by wake-up time
Start from the time you need to get up, count back whole 90-minute sleep cycles, then take off about 15 minutes to fall asleep. Five cycles is 7½ hours and six is 9 hours, which fits the 7 to 9 hours most adults need. For a 7:00 AM alarm, that means getting into bed at 9:45 PM or 11:15 PM.
Why sleep in 90-minute cycles?
Sleep isn't one long, even stretch. Through the night you go round a cycle: light sleep, then deep sleep, then dream (REM) sleep, and back to light sleep again. One trip round takes about 90 minutes on average, and a full night is usually four to six of them.
Being woken in the middle of deep sleep is what tends to leave you groggy (sleep inertia). Waking near the end of a cycle, when sleep is lighter, usually feels easier. So rather than picking a bedtime at random, it makes sense to aim for a whole number of cycles.
How to work out your bedtime
- Start with your wake-up time. It's the one fixed point on most days.
- Count back whole cycles of 90 minutes. Six cycles is 9 hours, five is 7½ hours, four is 6 hours.
- Take off the time it takes you to fall asleep. Most people take 10 to 20 minutes; 15 is a sensible default.
Worked example
Alarm at 6:30 AM. Five cycles back is 7½ hours, which is 11:00 PM. Take off 15 minutes to fall asleep: get into bed at 10:45 PM. For six cycles, it's 9:15 PM.
Going to bed now instead? Do it the other way round: add 15 minutes and then whole cycles. In bed at 11:00 PM, asleep by 11:15 PM, and five cycles later is 6:45 AM.
Bedtime table for common wake-up times
Each time already includes 15 minutes to fall asleep. Five or six cycles is the target for most adults.
| Wake up at | 6 cycles (9 h) | 5 cycles (7½ h) | 4 cycles (6 h) |
|---|---|---|---|
| 5:00 AM | 7:45 PM | 9:15 PM | 10:45 PM |
| 5:30 AM | 8:15 PM | 9:45 PM | 11:15 PM |
| 6:00 AM | 8:45 PM | 10:15 PM | 11:45 PM |
| 6:30 AM | 9:15 PM | 10:45 PM | 12:15 AM |
| 7:00 AM | 9:45 PM | 11:15 PM | 12:45 AM |
| 7:30 AM | 10:15 PM | 11:45 PM | 1:15 AM |
| 8:00 AM | 10:45 PM | 12:15 AM | 1:45 AM |
| 8:30 AM | 11:15 PM | 12:45 AM | 2:15 AM |
| 9:00 AM | 11:45 PM | 1:15 AM | 2:45 AM |
Use a 24-hour clock? 9:45 PM is 21:45, 11:15 PM is 23:15 and 12:45 AM is 00:45. Sleep Calculator shows times in whichever clock your phone uses, and works for any wake-up time and any time to fall asleep.
How much sleep do you need?
These are the general recommendations from the US National Sleep Foundation's expert panel (2015). For babies and young children, the hours include naps. The NHS gives similar guidance: most adults need 7 to 9 hours a night.
| Age | Hours a day |
|---|---|
| Newborns (0–3 months) | 14–17 |
| Infants (4–11 months) | 12–15 |
| Toddlers (1–2 years) | 11–14 |
| Preschool (3–5 years) | 10–13 |
| School age (6–13 years) | 9–11 |
| Teenagers (14–17 years) | 8–10 |
| Young adults (18–25 years) | 7–9 |
| Adults (26–64 years) | 7–9 |
| Older adults (65 and over) | 7–8 |
These are ranges, not targets to hit to the minute. If you wake up without an alarm on free days and feel alert through the afternoon, you are probably getting enough.
How long should a nap be?
- Power nap: about 20 minutes. You stay in light sleep, so you wake up refreshed rather than groggy. Set the alarm for 20 minutes plus the time it takes you to drop off.
- Full-cycle nap: about 90 minutes. One complete cycle, including deep and dream sleep, ending in light sleep. Useful after a short night.
- Avoid 30 to 60 minutes if you can: that's when you are most likely to be in deep sleep when the alarm goes.
Nap earlier in the day (before about 3 PM) so it doesn't push back tonight's bedtime.
Using Sleep Calculator
Sleep Calculator does the counting for you:
- Wake up at: set your alarm time with the big hour and minute dials (or a quick 6:00 to 8:00 chip) and see bedtimes for six, five and four cycles, with five and six marked as recommended.
- Sleep now: it takes the time on your phone, adds the time to fall asleep, and lists the best alarm times.
- Nap: alarm times for a 20-minute power nap and a 90-minute full cycle, starting now.
- Time to fall asleep: 15 minutes by default; change it in steps of 5 if you usually take longer.
It shows a 12- or 24-hour clock to match your phone, works offline, and stores nothing but your settings. It doesn't set alarms itself: pick a time and set it on your phone's alarm.
Tips for an easier morning
- Keep the wake-up time steady, weekends included. A fixed alarm is what sets your body clock.
- Get daylight soon after waking, and keep the last hour before bed dim and screen-light.
- Go easy on caffeine after lunchtime and on alcohol in the evening; both disturb the cycles later in the night.
- Can't sleep? If you're still awake after about 20 minutes, get up and do something calm in low light, then go back when you feel sleepy.
Questions
What time should I go to bed to wake up at 7 AM?
For six 90-minute cycles (9 hours) get into bed at 9:45 PM; for five cycles (7.5 hours) at 11:15 PM. Both allow 15 minutes to fall asleep.
How long is a sleep cycle?
About 90 minutes on average, but it varies from roughly 70 to 120 minutes between people and through the night. That's why a sleep calculator is a guide, not an exact science.
Is 6 hours of sleep enough?
For most adults, not regularly. General guidance is 7 to 9 hours a night for adults. Six hours (four cycles) now and then is fine, but a regular short night adds up.
How long should a nap be?
Either about 20 minutes, which keeps you in light sleep, or a full 90-minute cycle. Naps of 30 to 60 minutes are more likely to wake you from deep sleep and leave you groggy.
What if I can't fall asleep at the suggested time?
Don't lie there watching the clock. Keep the same wake-up time and aim for the next bedtime on the list. If you regularly struggle to sleep, talk to a doctor.