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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
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
The REM sleep discovery:
- Suggested that sleep is a
complex activity, fundamentally different from waking, but just
- 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
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
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.
When people sleep, they experience periods of Rapid Eye Movement. During this stage, which is associated with dreaming, the brain becomes very active. REM sleep and dreaming are triggered by the pons and neighboring structures in the brainstem. The pons sends signals to the thalamus and the cerebral cortex -- which is responsible for most cognitive activities. The pons also sends signals to shut off the neurons in the spinal cord.
A larger, higher resolution version of the graphic is available here. (154k)
Illustration by Lydia Kibiuk, Copyright © 1994 Lydia Kibiuk.
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