The phenomenon of sleep includes both passive mechanisms—for example, withdrawal of all external and
internal stimulation—and active mechanisms. Sleep is not simply an absence of wakefulness and perception,
nor is it just a passive phenomenon resulting from withdrawal of all sensory stimuli. Many areas of the brain remain active during sleep. All the structures responsible for generating sleep and wakefiilness are located within the brain. Sleep contributes to your overall health, affecting not only the brain but also the entire body, as evidenced by the many diseases that may result from too much sleep, too little sleep, inappropriate timing of sleep, or abnormal movements and behaviors intrucling into sleep. There are many chemicals sending signals to different groups of nerve cells in the brain switching off or switching on certain groups of neurons detemtining the sleep and wakefulness.
The word “sleep” is derived from the Latin word somnus, the German word slaf, and the Greek word hynos. Although sleep has aroused the interest of mankind since time immemorial, as reflected in the writings of the eastern and western religions and civilizations, the mystical nature of sleep and the definition of sleep have eluded the philosophers, religious scholars, scientists, and poets throughout the ages. Most realistically, this question of what sleep is should be asked when one is trying to get to sleep. But it is difficult to ask this question when we lie down, close out eyes, and cry to forget about the world of wakefulness. Slowly we begin drifting to a state beyond wakefulness when we do not see, hear, or perceive things in a rational or logical manner. We are now in another world, where we have no control, our brain cannot respond logically and adequately, and our body is relatively immobile. We are now entering what is termed predormitum. Soon we are drifting from lighter to deeper stages of sleep. We are now unconscious.
Fortunately, this state is reversible—a characteristic that differentiates sleep from irreversible coma {complete unconsciousness as a result of a disease) and death. Sleep in fact is referred as the brother of death in the classic literature (Homefs lliad, circa 700 B.C.). We will wake up to see the world of wakefulness after about seven or eight hours of sleep. Most of us go to sleep at night (unless we work a night shift). We finish our day`s activities, and after relaxing in the evening, we prepare to go to sleep. It is interesting to note, as Roger Eldrch wrote in his fascinating book At Day's Clare, that centuries before the discovery of light and electricity, night sleep habits of mankind were different from the consolidated sleep we seek nowadays. in the days past, there were two periods of sleep at night consisting of four hours at a stretch with a break of two to three hours in between. This interval was occupied with planning, dreaming, intimacy, meditating, and visiting.
Humans, animals, and plants all follow a basic rest activity pattern that occurs in cycles synchronized with
fluctuations of darkness and sunlight and is a Fundamental rhythmicity in all living organisms. The rotation of the Earth determines the timing of both the rest-activity cycle and the sleep-wakefulness rhythm.
Sleep—wake habits are oontrolled not only by external light and darkness as determined by sunrise and sunset but also by our internal body clock. This question of an intemal body clock was first raised more than two and a half centuries ago by a French astronomer named de Mairin. He noticed that the leaves of a heliotrope plant would open at sunrise and close at sunset, even when the plant was kept inside away from sunlight.
This observation led de Mairin to conclude that an internal clock in the plant must control the opening and closing of the leaves. Only toward the end of the twentieth century did scientists discover the existence of an internal clock in rats and shortly thereafter confirm that such a clock operates in humans as well. This human internal clock is thought to reside within a cluster of nerve cells (called suprachiasmatic nuclei) located deep in the center of the brain above the pituitary gland, the organ that is responsible for secretion of several important hormones. This internal clock has widespread connections, not only with the retina (nerve cell layer in the back of the eye) for receiving light from the outside world but also with other parts of the nervous system. As a result, it controls the body`s sleep—wake cycle, secretion of hormones, and the temperature rhythms.
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