Sleep cycle math: where the 90-minute rule comes from

What a sleep cycle is

Sleep is not one steady state. According to the National Institute of Neurological Disorders and Stroke (NINDS), you cycle through non-REM and REM sleep several times during a typical night. REM sleep first occurs about 90 minutes after falling asleep.

Each cycle starts in light non-REM sleep, moves into deeper stages, then reaches REM. Sleep Education describes the first stage, N1, as a brief stage of light sleep that lasts about 10 minutes or less. At the end of a cycle, you start over and re-enter non-REM sleep.

The formula

To find a bedtime, start at the wake time, take away the minutes it takes to fall asleep, then take away a whole number of cycles. To find a wake time, run the same steps forward from the moment you go to bed.

The calculator lists 6, 5, 4 and 3 cycles. At 90 minutes each, that is 9, 7.5, 6 and 4.5 hours of sleep.

The worked example

Say you want to wake at 07:00 and it takes you 15 minutes to fall asleep, the calculator's default.

• Forward, from a bedtime of 23:00: asleep at 23:15, then 5 cycles end at 06:45 and 6 cycles at 08:15.
• Change the fall-asleep time to 30 minutes and every bedtime moves 15 minutes earlier.

Why 90 minutes is an average

The 90-minute figure is a convenient round number, not a fixed length. Sleep Education states that each complete sleep cycle lasts about 90 to 110 minutes. It adds that most adults go through about four to six cycles in a full night.

Cycles also change across the night. NINDS describes increasingly longer, deeper REM periods occurring later in the sleep session. So a cycle at 05:00 is not a copy of the one at midnight.

That range adds up. If your cycles averaged 105 minutes, 5 cycles would be 8 hours 45 minutes, not 7.5 hours. Counting back from 06:45 would give 22:00, not 23:15. A calculator can't know your cycle length, so it uses the average.

Cycles are not the body clock

It is easy to confuse sleep cycles with the circadian clock. They are separate. NINDS explains that two internal biological mechanisms, circadian rhythm and homeostasis, work together to regulate sleep and wakefulness.

The circadian rhythm governs daily rhythms. It is driven by clusters of thousands of cells in the brain, the suprachiasmatic nucleus, that receive information about light exposure directly from the eyes. Sleep-wake homeostasis keeps track of your need for sleep.

Sleep cycles are the shorter loops within one night's sleep. The calculator works only with those loops. It does not model your body clock, so it can't say which bedtime suits your rhythm.

How much sleep, by the published numbers

Cycle counts say nothing about how much sleep is enough. For that, the published recommendations are the reference. NINDS states that most adults require 7 to 9 hours of sleep at night. The American Academy of Sleep Medicine and the Sleep Research Society recommend that adults aged 18 to 60 sleep at least 7 hours each night, as reported by the CDC.

In the worked example, 5 cycles (7.5 hours) and 6 cycles (9 hours) fall within 7 to 9 hours. Four cycles (6 hours) and three (4.5 hours) fall short of 7.