Home » Partners & Parents » Developmental Psychology » Sleep Stages

Sleep Stages

© Tara Kuther

Sleep has long been a mystery to mankind. Over the past 70 years we have learned a great deal about sleep, but puzzles remain. Most of what we have learned about sleep has come from research conducted in sleep labs, by researchers in the fields of psychology, psychiatry, and neurology.

In 1929, the invention of the electroencephalograph (EEG) made it possible to study the living brain non-invasively. The EEG has helped us learn much about sleep. Perhaps the biggest advance in studying sleep was the observation that the eyes of sleeping people move rapidly. Researchers refer to these periods when the eyes move back and forth as Rapid Eye Movement (REM) sleep.

How Do We Study Sleep?

Sleep research is conducted in sleep labs. Volunteers are recruited to sleep under study. They arrive an hour or two before bedtime. Electrodes that measure muscular activity and physiological changes are attached to various parts of the body. Electrodes record eye movements, and those attached to an EEG measure brain waves. The volunteer sleeps in a soundproof room where all movements, sounds, and body activities are recorded. As you might imagine, the first night's sleep is atypical under these conditions. The data from the first night is usually discarded. Several nights of data may be needed before the volunteer sleeps comfortably and normally.

Stages of Sleep

Typically we pass through five stages of sleep, in a cyclical fashion throughout the night. As we begin to fall asleep, we enter Stage 1 sleep. Stage 1 is a very light form of sleep, a sort of transitional phase between waking and sleep. Stage 2 sleep is characterized by the appearance of sleep spindles, short bursts of brain activity recorded by the EEG. Stages 3 and 4 are known as slow wave sleep, the deepest forms of sleep. It is difficult to wake someone from slow wave sleep, when we awake from this form of sleep we are often groggy and disoriented.

Collectively, Stages 1 through 4 sleep are referred to as non-REM sleep. During approximately the first 90 minutes of sleep, we cycle through the sleep stages: from Stage 1 to 2, 3, 4, and then back to 3, and 2. Instead of returning to Stage 1 sleep, we enter a different form of sleep, REM sleep.

During rapid eye movement (REM) sleep, the sleeping person exhibits rapid eye movements, high brain activity, and muscle paralysis. REM sleep is often referred to as paradoxical sleep because the very active brain is contrasted with the deeply relaxed body. During REM sleep the body is so relaxed that it is not possible to move. During REM sleep, we dream very vividly and imaginatively. After a period of REM sleep, we move back into Stage 2 sleep, and the cycle continues.