Sleep Cycles and Bedtimes: Working Out When to Sleep

What a sleep cycle is

A typical cycle moves from light sleep (stages N1 and N2) into deep sleep (N3, also called slow-wave sleep) and then into REM sleep, before starting again. In healthy adults a cycle averages about 90 minutes, though the real figure varies from person to person and from cycle to cycle, commonly somewhere between 80 and 120 minutes.

REM sleep is the stage most associated with vivid dreaming, when the brain is active while most of the body's muscles stay relaxed. Deep sleep is the stage that is hardest to wake from, and light sleep makes up the largest share of a typical night.

The mix shifts through the night. Deep sleep is concentrated in the earlier cycles, while REM periods grow longer towards morning. That is one reason why cutting the end of the night short tends to cut into REM sleep.

Counting back from your alarm

Most people also need some time to fall asleep. Calculators typically allow around 15 minutes for this, which is within the normal range for healthy adults. Then:

bedtime = wake time − (cycles × 90 min) − 15 minWork on a 24-hour clock; if the result goes below 00:00, add 24 hours.

Worked example: you need to wake at 06:30. Five cycles is 5 × 90 = 450 minutes, or 7 hours 30 minutes. Add 15 minutes to fall asleep, giving 7 hours 45 minutes. Counting back from 06:30 gives a bedtime of 22:45.

Adjust the final term to suit yourself. If you usually need 30 minutes to fall asleep and your alarm is at 07:15, five cycles plus 30 minutes is 450 + 30 = 480 minutes, or exactly 8 hours, so the bedtime is 23:15.

Here are the options for the same 06:30 alarm:

Small errors in the cycle length add up. If your cycles actually last 100 minutes rather than 90, five of them take 500 minutes, not 450. With the 22:45 bedtime and a 06:30 alarm you would have slept 450 minutes, which is only 4.5 of your cycles, so the alarm would land in the middle of the fifth one. This is why a cycle-based bedtime is best treated as a starting point to adjust after a week or two of noting how you feel.

Counting forward from bedtime

You can also go the other way. If you get into bed at 23:00 and expect to fall asleep by 23:15, add whole cycles to 23:15:

If your alarm is fixed at 06:30, it falls 15 minutes before the end of the fifth cycle, which is close enough. The point of the exercise is to choose a bedtime that gives you enough total sleep, not to hit a perfect boundary.

What the method cannot do

• Cycles are not exact. Your own average may be shorter or longer than 90 minutes, and it changes during the night, so the method gets you into the right region, not to the minute.
• Total sleep matters most. Most adults are advised to get roughly 7 to 9 hours. Four cycles (6 hours) falls below that range for most people, even though it is a whole number of cycles.
• Waking from deep sleep feels worse. Being woken during deep sleep can leave you groggy for a while, which is the idea behind timing alarms for the end of a cycle. The effect is real but modest compared with simply being short of sleep.
• Falling asleep takes varied time. If you usually take 30 minutes, use 30 in the formula. Falling asleep within a couple of minutes every night can be a sign of accumulated sleep debt.
• Consistency helps. A steady schedule throughout the week usually serves you better than hunting for the perfect bedtime each night.

To try different wake-up times and latencies, use the sleep calculator, or add fixed durations to a time with the add hours calculator.

This article is general information, not medical advice. If you regularly struggle to fall asleep, wake unrefreshed despite enough hours, or feel very sleepy in the daytime, speak with a doctor.