Sleep is a state that can be easily defined as a cyclic period of all animals in which they stay at rest and do not respond easily to external stimuli. Intuitively we can guess that sleep is necessary to recover energy by resting. Although this is partly true, during sleep many things happen in the brain that suggest that sleep is less a resting state and more an active part of the normal functioning of the organism.

In order to distinguish between a sleeping person and somebody just resting with his or her eyes closed, scientists have used the electroencephalogram (EEG) to study the states of wakefulness and sleep in an objective way. More than 60 years ago they discovered that sleep is composed by 2 easily distinguished stages or phases. One is characterized by slow EEG activity, general relaxation but with sporadic movements of the extremities or the whole body, and deep breathing, and is usually referred to as slow wave sleep; while the other stage shows very fast EEG activity, similar to attentive wakefulness, body flacidity except for distal extremities, and the eyes move in rapid successions. This stage is known as rapid eye movement (REM sleep) or paradoxical sleep, due to the similarity between the brain activity of this sleep and wakeness. Although REM sleep is less than a third of the total time we sleep per night, it is the most studied phase. This is probably due to the fact that most dreaming seems to happen during REM sleep, and specific deprivation of this kind of sleep disturbs normal functioning in a way similar to full sleep deprivation, even if total sleeping time (only in slow wave sleep) is normal or even greater than normal.

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