Sleep and dreaming
The majority of animals, including humans, experience what are called 'circadian cycles,' which are daily rhythms of activity and rest. These cycles are not responses to external conditions, like daylight and darkness, but are generated by an internal, biological clock. Experiments where animals lived in artificial surroundings showed the persistence of the circadian cycle even in the absence of daylight or temperature variations.
Daily resting periods serve to spare the animal's resources for the time of day when it can make best use of them, and to make the animal less noticeable to its predators. In high-level animals, as birds and mammals, a resting period can become a sleep period, which is a special state that is not only a period of diminished activity.
Sleep, which occurs only in animals with complex nervous systems, can be of two very different kinds. These kinds can be identified observing the sleeping animal or, with greater precision, observing the brain electric waves by means of an electroencephalograph. The most detectable difference, apart from brain waves, is the presence or absence of rapid eye movements (REM) and dreams. Thus, the two kinds of sleep--that differ between them as much as from the awake state--have become to be known as REM sleep and non-REM sleep.
In REM sleep, the person exhibits high activity in the brain cortex, and eyes that move coordinately as if the person were watching something; if the person is awakened, generally reports to have been dreaming. In non-REM sleep, brain activity is lessened, eye movements are slow or do not exist, and people report fewer dreams. In addition to these main variations many other bodily functions differences exist between these two forms, which alternate during the time that the individual is asleep. He can have several dreams in a night, but most of them are not recalled when he awakes.
It is supposed that REM and non-REM sleep have dissimilar purposes, but what are they it is not known. Wakefulness and the two forms of sleep are produced by different parts of the central nervous system. Sleep is originated in neurons located in the brain stem, while wakefulness arises from the reticular formation. The cycles of REM and non-REM over a night originate from the interaction of two groups of neurons that communicate with each other using neurotransmitters. Another region of the brain stem is responsible of regulating daily rhythms of activity and rest, sleep and wakefulness, temperature, and hormone levels.
Sleep is essential to life. Losing a few hours of sleep can diminish the efficacy to perform mental tasks and curtail the ability to make sensible judgments. The need for sleep varies among individuals and also according to age. The newborn spends two thirds of the day in sleeping, in several periods. As he grows up, he tends to sleep less and to do it principally at night. The adult human spends one third of his time sleeping, mainly during the night. A nap ('siesta') in the mid-afternoon is customary in many cultures. At old age, the person usually sleeps less at night and more in the daytime, and in shorter periods.
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