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levels of brain activity, sleeping person, long necks, armadillos, particular positions

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Sleep, natural state of rest characterized by reduced body movement and decreased awareness of surroundings. Sleep is distinguished from other sleeplike states, for instance, hibernation or coma, because it is easily interrupted by external stimulation, such as a loud noise. While the exact purpose of sleep remains a mystery, sleep researchers have made enormous strides in understanding how sleep occurs in humans and other animals, and the nature of sleep disorders.

All mammals and birds sleep, but scientists are unsure if reptiles, fish, insects, and other life forms sleep. Total sleep amounts differ greatly across species. In general, large mammals tend to sleep less than small mammals. The giraffe and elephant, for instance, sleep only 2 to 4 hours a day, while bats, opossums, and armadillos sleep 18 hours a day or more.

While sleeping, most animals close their eyes and adopt particular positions referred to as sleep postures. Humans typically lie down to sleep, for example, while giraffes kneel and bend their long necks around to rest their heads in the crook of their hind knee. Some animals, such as dolphins, can sleep while they are moving.

Scientists measure sleep by placing metal electrodes on the scalp to record the electrical activity of the brain. This procedure, called electroencephalography (EEG), enables sleep researchers to evaluate levels of brain activity at different times during sleep. Researchers use similar electrodes to record a sleeping personís body muscle activity and rate of eye movement.


Siegel, Jerome M., B.S., Ph.D.

Professor, Department of Psychiatry, University of California, Los Angeles; Chief of Neurobiology Research, Sepulveda Veterans Affairs Medical Center, Los Angeles. Associate Editor, ''Sleep''.

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