What are sleep cycles?

The term "sleep cycle" refers to the sequence of stages we pass through as we sleep. Typically, a sleeper repeats the cycle several times each night.

Sleep cycles have been observed in many species of mammal and birds. They have also been detected in some reptiles, like the lizard called the "Australian dragon." But here I will focus my discussion on sleep cycles in human adults. Sleep cycles in other species -- and in human infants -- look somewhat different.

The stages of sleep have been defined primarily by electrical brain activity detected via electroencephalograms, or EEGs. If you want to understand what a sleep cycle is, it's helpful to know a little about this electrical activity.

The EEG measures changes in voltage caused by patterns of neural firing in the brain. When we're awake, most of our neurons fire according to their own, varied rhythms. But as we fall asleep, more neurons begin to fire in synchrony with each other. This gives rise to a distinctive EEG pattern: It produces electrical waves with higher peaks and valleys, or higher amplitude. In addition, as we sink into deeper stages of sleep, neurons also fire at a slower rate. This gives rise to another characteristic that we can see on an EEG readout-- the brain waves have a lower frequency.

When we pass out of the deepest stage of sleep (slow-wave sleep), we typically move into a strange stage called "rapid eye movement" (REM) sleep, so-named because the sleeper's eyes shift back and forth rapidly under the lids. During REM, the muscles slacken, and the body undergoes a temporary paralysis. But what's really odd or paradoxical about REM is the EEG pattern: The neurons become less synchronized, much as they do when we're awake. REM is one of the ongoing scientific puzzles of sleep, but there is no doubt this is a form of sleep. And it's important to realize that not all parts of the brain look active during REM. Some parts of the brain are characterized by slow waves.

What exactly does the typical sleep cycle look like?

In an adult sleeper settling down for the night, a single cycle is composed of 3 stages of non-REM sleep (in older texts, you may see reference to 4 stages) and REM. The whole cycle takes an average of 70-100 minutes the first time through, at which point the sleeper either awakens or begins the cycle again. Subsequent cycles can be longer -- between 90-120 minutes. And each time you repeat a cycle, things aren't exactly the same. You usually spend more time in the third stage of sleep during your first sleep cycle. In subsequent cycles, you spend less time in that stage and more time in REM. In addition, individuals vary in how much time they spend in each stage. Most adults experience between 3-5 cycles each night.

Let's take a closer look at each stage. I will abbreviate non-REM sleep as "NREM."

NREM1 

This is the lightest stage of sleep, and in some cases people in NREM1 maintain a sense of awareness of their surroundings. You are very easily awakened from NREM1. For instance, you will likely awaken if someone speaks your name. But scientists can identify this as sleep because your brain waves are a bit slower than during waking. Typically, the stage lasts between 1-7 minutes.

NREM2

This is still considered light sleep, but the waves have slowed further, which indicates that you are sleeping more deeply than before. Occasionally your EEG shows brief bursts of slower, high amplitude waves (k-complexes). These may function, in part, to keep you from awakening to environmental sounds. This stage lasts about 10-25 minutes.

NREM3 (formerly divided into two stages, NREM 3 and 4)

This is the stage where brain waves become the slowest. Such sleep is therefore called slow-wave sleep. This is when you are least likely to be awakened by environmental noise. The stage lasts between 20-40 minutes.

NREM 2 (again)

People usually return to this stage for a few minutes before transitioning to REM.

REM

The length of REM varies a great deal depending on how many times the cycle has been repeated. The first time through, a sleeper might spend little or no time in REM. In later cycles, the sleeper might spend much more time in REM than in NREM3. Overall, adults spend between 20-25% of the night in REM.