Planning bedtimes around sleep cycles, and how much sleep you need

Guide · Updated 2026-11-10

What a sleep cycle is, how counting in cycles picks a bedtime or an alarm time, how much sleep experts suggest by age, and a worked example with a custom cycle length.

Open the Sleep calculator Free, in your browser, nothing uploaded.

The sleep calculator suggests bedtimes that fit a wake-up time, or alarm times that fit a bedtime, by counting in whole sleep cycles. You pick what you know, adjust how long you take to drop off and how long your cycles last, and it lists three times, with a table of how many hours of sleep experts suggest for each age. It is for students with early classes, shift workers, parents planning a child’s evening, and anyone curious about sleep. It is general education only, not medical advice: if you are often tired after a full night, talk to a doctor. Travelling across time zones? The jet lag planner helps shift your sleep, and the study timer keeps evening revision from running into bedtime.

What it is

Through the night, sleep runs in repeating cycles. Each one moves from light sleep into deep sleep and then into REM (rapid eye movement) sleep, the stage linked with vivid dreams. A cycle lasts roughly 90 minutes on average, though it can be anywhere from about 70 to 120 minutes and changes as the night goes on.

Waking in light sleep near the end of a cycle often feels gentler than being pulled out of deep sleep. So instead of aiming for a set number of hours, the calculator counts whole cycles:

alarm = bedtime + time to fall asleep + n × cycle length

bedtime = wake time − n × cycle length − time to fall asleep

Here n is the number of cycles. The tool shows n = 6, 5 and 4. With 90-minute cycles that is 9 hours, 7 hours 30 minutes and 6 hours of sleep. Five or six cycles fit the 7 to 9 hours most adults are advised to get, so those two are highlighted. Four cycles is marked as a short night.

The cycle idea is a handy rule of thumb, not an exact science. Treat the times as a starting point and adjust them to how you feel.

How to use it

1. Choose a tab: I wake up at, I go to bed at, or Going to bed now (which reads the time from your device’s clock and hides the time box).
2. Set the wake-up time or bedtime.
3. Change Minutes to fall asleep if you usually drop off faster or slower than 15 minutes.
4. Change Cycle length (minutes) if you have found a length that suits you better than 90.
5. Pick 24-hour or 12-hour (AM/PM) in Clock.
6. Read the three times, each with its number of cycles and hours of sleep. Check the How much sleep by age table below them. Reset puts back the starting example.

A worked example

Omar has a 06:15 alarm for an early shift. He uses the I wake up at tab and keeps the defaults: 15 minutes to fall asleep and 90-minute cycles.

• 6 cycles: 6 × 90 = 540 minutes. 06:15 minus 540 minutes is 21:15, minus 15 more is 21:00.
• 5 cycles: 450 minutes back gives 22:45, minus 15 is 22:30.
• 4 cycles: 360 minutes back gives 00:15, minus 15 is 00:00.

He finds 22:30 realistic, giving 7 h 30 min of sleep.

After a few weeks he notices he often wakes on his own about 100 minutes into each cycle, so he sets the cycle length to 100. The times move to 20:00 (6 cycles, 10 h), 21:40 (5 cycles, 8 h 20 min) and 23:20 (4 cycles, 6 h 40 min).

The other way round: going to bed at 22:40, taking 20 minutes to fall asleep, with 90-minute cycles, the I go to bed at tab suggests alarms at 08:00, 06:30 and 05:00.

Where it is used

• School and exam weeks. Students can work back from the first bell to a bedtime that still gives 8 to 10 hours, the range suggested for teenagers.
• Shift work. Nurses, drivers and factory staff sleeping in the daytime can use the same sums for any start time.
• Parents. The age table helps plan a child’s bedtime, remembering that the ranges for young children include naps.
• Travel. Before a long flight, people use it to plan the night’s sleep around an early departure.
• Science class. It is a simple way to talk about sleep stages and why sleep is studied in cycles.

Common mistakes

• Reading the times as exact. Your cycles are not a clock. If an alarm still catches you in deep sleep, shift it by 10 to 15 minutes or change the cycle length.
• Leaving out time to fall asleep. If you lie awake 30 minutes most nights, set that number, or every time will be off.
• Picking four cycles every night. Six hours is a short night for most adults. Use it for the odd early start, not as a routine.
• Using cycles to fix ongoing tiredness. Feeling tired often, snoring loudly or waking gasping are reasons to see a doctor, not to change an alarm.
• Mixing up AM and PM. On the 12-hour clock, check that a bedtime reads PM before you rely on it.

Questions people ask

Can I count my naps in cycles too? Many people keep naps short, around 20 minutes, to avoid waking from deep sleep, or take a full cycle. The calculator is built for night sleep, so treat it as a rough guide for naps.

Why does going to bed now give different times each time I open it? That tab uses the current time on your device, so the alarms move forward as the clock does.

Do the age ranges apply everywhere? They come from US sleep medicine groups and are widely used. Your own doctor’s advice comes first.