A sleep cycle lasts about 90 minutes, and most adults need about 7 to 9 hours, which is five or six cycles. To pick a bedtime, count back in 90-minute steps from the time you need to wake, and add roughly 15 minutes to fall asleep.
Waking at the end of a cycle, rather than in the middle of deep sleep, often feels easier. The numbers are a guide, because cycle length varies from person to person and from night to night.
What is a sleep cycle?
During the night you move through lighter sleep, deeper sleep and REM (dream) sleep, then start again. One full loop averages about 90 minutes, though it can be shorter or longer for you. Deep sleep tends to dominate the early part of the night, and REM periods get longer towards the morning.
Being woken during deep sleep can leave you groggy, which is sometimes called sleep inertia. Timing your alarm near the end of a cycle may help, but it is no substitute for getting enough sleep overall.
How many hours do you need?
The NHS says most adults need between 6 and 9 hours, and a common rule of thumb is about 7 to 9. In cycle terms:
| Cycles | Time asleep |
|---|---|
| 3 | 4 h 30 min |
| 4 | 6 h |
| 5 | 7 h 30 min |
| 6 | 9 h |
Children and teenagers need more than adults. If you regularly feel tired, or need a lot more or less sleep than the guide suggests, speak to a GP.
How to calculate your bedtime
Use this simple method:
- Choose the time you need to wake up.
- Subtract 90 minutes for each cycle you want, usually five or six.
- Subtract another 15 minutes or so for falling asleep.
For a 7:00 am alarm and five cycles, that is 7 h 30 min of sleep plus 15 minutes to drop off, so you need to be in bed by 11:15 pm. Our Sleep calculator tool gives the same answer, plus the other options:
| Bedtime | Cycles | Sleep |
|---|---|---|
| 9:45 pm | 6 | 9 h |
| 11:15 pm | 5 (best) | 7 h 30 min |
| 12:45 am | 4 | 6 h |
| 2:15 am | 3 | 4 h 30 min |
Working out when to wake up
Going the other way works too. If you are going to bed at 11:00 pm and take 15 minutes to fall asleep, five cycles ends at 6:45 am and six cycles at 8:15 am. Four cycles would wake you at 5:15 am, which is a short night for most adults.
If you set an alarm for a time between cycles, you may not notice much difference. Cycle length shifts, so if you feel better waking ten minutes earlier or later, trust that.
Why the 90-minute figure is only a guide
The 90-minute average hides plenty of variation. Early cycles can be shorter, later ones longer, and age, stress, alcohol and caffeine all change how you move through the stages. Wearable devices that claim to track stages are not medical devices, so use them with caution.
A practical approach is to pick a target of five or six cycles, try it for a week or two, and note how you feel on waking. Adjust the bedtime by 15 or 30 minutes until you find what works for you.
Tips for better sleep
- Keep a regular bedtime and wake time, including at weekends where you can.
- Keep the bedroom cool, dark and quiet.
- Avoid caffeine late in the day. Our guide to How much water should you drink each day? also covers other drinks that count towards your day.
- Wind down without bright screens for a while before bed.
- Avoid heavy meals and alcohol close to bedtime.
- Exercise earlier in the day if late sessions keep you awake. See Running pace and race finish times explained if you are planning a training routine.
Sleep also affects appetite and energy. Our Calories and TDEE: how many calories do you need? guide looks at how much you eat and burn each day.
Note: This is general information, not medical advice. Our tool gives an estimate. If you struggle to sleep for weeks, snore heavily or feel unusually sleepy in the day, see a GP.
Try the calculator
Enter your wake-up time or bedtime into the Sleep calculator calculator to see the best times, including how long you usually take to fall asleep.
Open the free Sleep calculator
Worked example 2: an early start and a slow dropper-off
Say you need to be up at 6:30 am. Our sleep calculator gives these bedtimes, first with the default 15 minutes to fall asleep, then with 30 minutes:
| Cycles | Time asleep | Bed by (15 min to fall asleep) | Bed by (30 min to fall asleep) |
|---|---|---|---|
| 6 | 9 h | 9:15 pm | 9:00 pm |
| 5 | 7 h 30 min | 10:45 pm | 10:30 pm |
| 4 | 6 h | 12:15 am | 12:00 am |
| 3 | 4 h 30 min | 1:45 am | 1:30 am |
The only difference between the columns is the extra 15 minutes. If it regularly takes you half an hour to nod off, using the wrong setting means you will be short of sleep on most nights without realising it.
Now flip it round. If you get into bed at 11:30 pm with a 15-minute lead-in, the tool shows these wake times:
| Cycles | Wake at |
|---|---|
| 3 | 4:15 am |
| 4 | 5:45 am |
| 5 | 7:15 am |
| 6 | 8:45 am |
A five-cycle night here ends at 7:15 am, so if your alarm has to go off at 7:00 am you are waking about 15 minutes before the end of the fifth cycle, which is fine. Do not fret about being a few minutes off.
Step by step: counting back by hand
You can do this on paper in under a minute.
- Write down the time you must be awake, for example 6:30 am.
- Choose the number of cycles. Multiply by 90 minutes: five cycles is 450 minutes (7 h 30 min).
- Subtract that from the wake time: 6:30 am minus 7 h 30 min is 11:00 pm.
- Subtract your time to fall asleep. With 15 minutes, the bedtime is 10:45 pm.
To go the other way, add the cycles and the lead-in to your bedtime and read off the answer. When you are using this to plan around work, subtract your getting-ready and travel time first to get the wake time.
Sleep debt and why a lie-in does not fix everything
If you sleep five hours on weekdays, you build up a shortfall. Many people sleep for much longer at weekends to catch up, but swinging the schedule around can make Monday mornings harder, because your body clock shifts later and then has to shift back.
A gentler approach is to move your bedtime earlier by 15 minutes every few nights until you wake up before the alarm without effort. If you still need a long lie-in every weekend even after a week of earlier nights, you may be short of sleep, or something else may be going on. A GP can help.
Shift work, jet lag and the clocks changing
Cycle timings work best when your body clock agrees with your schedule. They are less reliable when you sleep during the day, after a time zone change or on irregular rotas. In those cases, use the calculator for the length of the night, not for finding a perfect moment.
In the UK the clocks go back on the last Sunday of October (25 October in 2026) and forward on the last Sunday of March. The week after the change, you may need to shift your bedtime by 15 to 20 minutes a night to adjust, rather than all at once. For longer journeys, see Time zones explained: how to convert times without mistakes.
What to do if the numbers do not fit your life
Sometimes the calculation says you should be in bed at 9:45 pm and that is simply unrealistic. You have options:
- Be honest about four cycles. Four cycles is six hours, which is at the bottom of or below the range most adults need. If you do this regularly, you may notice it in your mood and concentration.
- Move the alarm instead. If you cannot move bedtime, try to find an extra 30 minutes in the morning by preparing clothes and food the night before.
- Protect the wind-down. Even on a short night, a quiet last half hour with no screens can make falling asleep faster.
- Consider a short nap. A brief daytime rest can help some people, but a long late nap can make it harder to sleep at night.
Common mistakes
- Counting from lights out and forgetting the lead-in. You are not asleep the moment you lie down.
- Treating 90 minutes as exact. It is an average, and yours may differ.
- Relying on a wearable. Sleep-stage tracking on consumer devices is an estimate.
- Cutting sleep for exercise. If training makes you tired all day, see Running pace and race finish times explained and Heart rate zones explained: how to find your maximum and train by zone for pacing effort sensibly, and protect your sleep too.
- Using caffeine or alcohol as a fix. Both can alter sleep quality, even when you do fall asleep quickly.
Frequently asked questions
How long is a sleep cycle?
About 90 minutes on average, but it varies between people and through the night.
Is five cycles enough?
Five cycles is 7 hours 30 minutes, which falls within the range of about 7 to 9 hours most adults need. Some people feel better with six.
Do I need to wake at the end of a cycle?
No. It can help you feel less groggy, but total sleep matters more than exact timing.
What if I fall asleep faster or slower than 15 minutes?
Adjust the figure in the calculator. If you regularly take much longer than 30 minutes, talk to a GP.
Is it better to sleep six hours with a perfect cycle or seven without?
Total sleep matters more than neat timing. If you need to choose, aim for enough hours first, then adjust the alarm to a cycle boundary.
How many cycles should teenagers and children aim for?
They need more sleep than adults, so they usually need more cycles. Check NHS guidance for the recommended hours for each age group.
Why do I wake up before my alarm?
Your body clock is quite regular and may wake you shortly before a habitual time. If you wake feeling rested, it is not a problem. If you wake in the small hours and cannot get back to sleep regularly, speak to a GP.
Does the calculator work for a daytime sleep after a night shift?
It can show the length of sleep in cycles, but daytime sleep is often lighter and shorter. Make the room dark and quiet, and treat the timings as a rough guide.