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The discovery of rapid eye movement (REM) sleep, a mentally active period during which dreaming occurs, provided a biological explanation for this phenomenon. It also inspired interest in sleep research by giving scientists a marker for changes in the brain during sleep. From this knowledge, they have begun to understand and develop treatments for major sleep disorders such as insomnia and sleep apnea.
Everyone sleeps. This fundamental activity consumes one-third of our lifetimes and can overpower all other needs. But what does sleep do for us? What happens when you are sleep deprived? What are sleep disorders?
Much of what is known about sleep stems from the groundbreaking 1953 discovery of rapid eye movement (REM) sleep. This is an active period of sleep marked in humans by intense activity in the brain and rapid bursts of eye movements. At the same time, scientists discovered that REM sleep is when dreaming occurs.
Before the 1950s, most scientists thought of sleep as an unchanging, dormant period of little interest. Hardly anything was known about sleep or dreaming.
The earliest hints that sleep was a changing state came with studies showing that blood pressure, heart rate, and other body functions in humans rise and fall in a pattern during sleep. Because researchers had observed some eye movement during sleep, they recorded these movements by placing electrodes behind the eyes. They also recorded muscle activity and brain waves. They found regular periods of very rapid eye movement and rapidly changing brain waves that alternated with periods of deep, quiet, sleep marked by large, slow brain waves. Later, scientists found that the body is paralyzed during REM sleep.
The REM sleep discovery:
Suggested that sleep is a complex activity, fundamentally different from waking, but just as active.
Provided a biological marker for dreaming so that immediate dream reports could be collected.
Compelled scientists to examine the physiology of sleep.
When researchers woke people up during REM sleep and asked them about their dreams, they found that almost all who awakened during REM sleep could remember their dreams. They realized that people who claim they do not dream really do not remember their dreams the next morning. Also, scientists found that, rather than being fleeting events, dreams vary in length according to the length of REM period.
In later studies, scientists divided non-REM sleep into four stages, accounting for about 75 percent of total sleep. In each stage, brain waves become progressively larger and slower, and sleep becomes deeper. After reaching stage 4, the deepest period, the pattern reverses, and sleep becomes progressively lighter until REM sleep, the most active period, occurs. This cycle typically occurs about once every 90 minutes in humans.
Scientists found that brain activity during REM sleep begins in the pons, a structure in the brainstem, and neighboring midbrain regions. The pons sends signals to the thalamus and to the cerebral cortex, which is responsible for most thought processes. It also sends signals to turn off motor neurons in the spinal cord, causing a temporary paralysis that prevents movement.
Research on normal sleep led scientists to recognize and study sleep disorders, which afflict up to 70 million Americans. These disorders include insomnia, or difficulty in falling asleep, and sleep apnea, which causes breathing to stop for extended periods during sleep. These can cause behavior problems and accidents related to fatigue.
Once sleep disorders became recognized, scientists began to find treatments for them.
Almost everything known about the physiology of sleep has been learned by studying experimental animals. For example, scientists found that sleep phases are closely related to the activity of certain groups of nerve cells releasing brain chemicals that relay information from one neuron to another. Research on these specialized cell groups is helping scientists to devise specific drug treatments for sleep disorders.
Yet much remains to be uncovered. Exactly what sleep does for humans is unknown. Researchers are just beginning to unravel the mechanisms explaining why and how people nod off and wake up.
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