How the sleep calculator works
Through the night you move through repeated sleep cycles. Each one takes you from light sleep into deep sleep and then into REM (dreaming) sleep, and lasts roughly 90 minutes on average, though it can be anywhere from about 70 to 120 minutes. Most adults go through four to six cycles a night. Waking at the end of a cycle, when you're in lighter sleep, tends to feel easier than being woken from deep sleep.
The calculator counts back from your alarm (or forward from your bedtime) in whole cycles, and adds the time it takes to fall asleep. Sleep experts say 10 to 20 minutes is normal, so the calculator uses 15 minutes unless you change it:
bedtime = wake-up time − time to fall asleep − (cycles × 90 minutes)
wake-up time = bedtime + time to fall asleep + (cycles × 90 minutes)
It highlights the option that gives you between 7 and 9 hours, in line with the NHS.
How much sleep do adults need?
The NHS says most healthy adults need around 7 to 9 hours of sleep a night, although the exact amount is different for each person. Five cycles of 90 minutes is 7 hours 30 minutes and six cycles is 9 hours, so both fall inside that range. Four cycles, 6 hours, is short for most adults if it happens night after night.
Children and teenagers need more sleep than adults, and the amount changes with age, so this calculator is designed for adults.
Worked examples
- Waking at 7:00am: 7:00 minus 15 minutes to fall asleep is 6:45. Take off five cycles (7 hours 30 minutes) and the bedtime is 23:15 (11:15pm). For six cycles (9 hours), go to bed at 21:45.
- Waking at 6:30am: five cycles means going to bed at 22:45; four cycles would be 00:15 but only gives you 6 hours.
- Going to bed at 10:30pm: you're likely to be asleep by 22:45. Five cycles later it's 06:15, and six cycles later it's 07:45.
Bedtimes for common wake-up times
| Wake-up time | 6 cycles (9h) | 5 cycles (7h 30m) | 4 cycles (6h) |
|---|---|---|---|
| 5:30am | 20:15 | 21:45 | 23:15 |
| 6:00am | 20:45 | 22:15 | 23:45 |
| 6:30am | 21:15 | 22:45 | 00:15 |
| 7:00am | 21:45 | 23:15 | 00:45 |
| 7:30am | 22:15 | 23:45 | 01:15 |
| 8:00am | 22:45 | 00:15 | 01:45 |
These allow 15 minutes to fall asleep. Treat them as a guide, not a rule: sleep cycles aren't exactly 90 minutes, and they tend to get longer and contain more REM sleep later in the night.
Tips for sleeping better
- Keep a routine. The NHS recommends going to bed and getting up at roughly the same times every day, including weekends.
- Wind down. Give yourself time to relax before bed, for example with a bath, reading or gentle music, and put your phone away.
- Make your bedroom sleep-friendly. Dark, quiet, tidy and cool usually works best.
- Watch caffeine and alcohol. The NHS advises avoiding alcohol, tea, coffee and cigarettes for at least 6 hours before bed. The alcohol units calculator shows how much you drink in a typical week.
- Time your exercise. Being active helps you sleep, but try to avoid exercise in the 4 hours before bed. If you run, the running pace calculator can help you plan sessions.
- Don't lie there worrying. If you can't sleep, get up and do something relaxing, then go back to bed when you feel sleepy.
Shift workers can still use the calculator: just enter the time you need to get up, whatever time of day it is. To add up the hours you work across shifts, try the hours calculator.
If you've had trouble sleeping for months, if poor sleep is affecting your daily life, or if a partner says you snore loudly or stop breathing in your sleep, speak to your GP. Insomnia and sleep apnoea are common and can be treated.
Frequently asked questions
What time should I go to bed if I wake up at 7am?
For five 90-minute sleep cycles (7 hours 30 minutes) plus 15 minutes to fall asleep, go to bed at 11:15pm. For six cycles (9 hours), go to bed at 9:45pm.
How many hours of sleep do adults need?
The NHS says most healthy adults need around 7 to 9 hours of sleep a night, although the exact amount varies from person to person.
How long is a sleep cycle?
A sleep cycle lasts about 90 minutes on average, but it can range from around 70 to 120 minutes. Most adults have four to six cycles a night.
How long should it take to fall asleep?
Around 10 to 20 minutes is normal. If it regularly takes you more than half an hour and you feel tired during the day, speak to your GP, as it could be insomnia.
Is 6 hours of sleep enough?
For most adults, regularly getting 6 hours is less than the 7 to 9 hours the NHS recommends. Some people can manage on less for a night or two, but it's not ideal long term.
Last reviewed October 2026 using 2026/27 figures. This calculator gives an estimate for guidance only.