Chapter 1. Stages of Sleep

1.1 Stages of Sleep

How Do Researchers Measure the Stages of Sleep? Sleep researchers use the brain's weak electrical signals, generated by the simultaneous firing of millions of neurons, to measure sleep stages. Electrodes placed on the scalp can detect these signals and provide scientists with a permanent record—called an electroencephalogram (EEG)—of the gross activity of the brain across an entire night of sleep. Although the shape of the EEG wave changes gradually, minute by minute, during the course of a night’s sleep, researchers have divided the sequence into five stages.

1.2 Stages of Sleep

Stage 1 Sleep: the transitional stage between wakefulness and sleep, associated with imagery resembling hallucinations.

Stage 2 Sleep: the stage of light sleep characterized by brief bursts of brain activity called sleep spindles.

Stage 3 Sleep: the stage of deep sleep in which delta waves (large, slow brain waves) make up 10 to 50 percent of the EEG record.

Stage 4 Sleep: the stage of deep sleep in which the EEG record consists almost entirely of delta waves (large, slow brain waves).

REM Sleep: the sleep stage in which the brain and eyes are active, the muscles are relaxed, and vivid dreaming occurs.

1.3 Stages of Sleep

In a typical night, you pass through each of these stages of sleep several times in a regular pattern called the sleep cycle. Notice that most of the slow-wave, deep sleep (Stages 3 and 4) occurs in the first half of the night, while the longest REM sleep periods are later in the night.

1.4 Explain. Stages of Sleep

You will spend about one-third of your life asleep. Have you ever wondered what your brain is doing while you are sleeping? In the 1950s, researchers discovered that the brain gives off weak electrical signals, generated by the simultaneous firing of millions of neurons. Electrodes placed on the scalp can detect these signals, which provide a graphic record—an electroencephalogram (EEG)—of the overall activity of the brain. When sleep researchers examined the EEG "brain waves" across an entire night’s sleep, they found a clear pattern: Slower waves are associated with greater relaxation. Brain activity gradually slows as a person drifts off to sleep, then gets steadily slower and more regular over the next half-hour or so, as the person moves through four stages of deeper and deeper sleep. This pattern then reverses, and the person moves back through Stages 3 and 2. But instead of returning to Stage 1, the sleeper goes into a very different stage called REM sleep, accompanied by jerky eye movements and vivid dreams. This sequence (dipping into deeper sleep, then lighter sleep followed by REM sleep) is called the sleep cycle, and is repeated 3 to 5 times in a typical night.

1.5 Question

This is an example of a tracing from Stage 4 because it consists almost entirely of delta waves.

This is Stage 2 because it consists of moderately slow waves interspersed with bursts of rapid activity.

REM sleep has fast, small waves similar to those from a person who is awake and alert.

Stage 3 has moderately slow waves interspersed with very slow, very large delta waves.

This example is Stage 1, showing a mixture of small, fast waves and slightly slower waves. It also has some similarity to a REM sleep tracing.

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