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Sleep is a complex physiological state, necessary and inherent to human existence. Although it is a universal function of the brain, there are still many unsolved mysteries that captivate specialists of all disciplines. The enigma, that accompanies the dream, the suspension of the consciousness, the experience of dreaming, and the inability to function without sleep, have fascinated poets and philosophers for thousands of years.
Sleep is a rest state, that uses the bodys to recover from the sustained efforts during the period of wakefulness (wakefulness). It is a transient and reversible disconnection with the environment, detectable by changes in brain waves. This period occupies one third of our lives and beyond simply being asleep, it is a biological and behavioral state with very specific features. Finding a scientific definition of what is really a dream is not an easy task. Each author presents his unique insights to define briefly the complexity of this phenomenon. For Kaplan and Sadock sleep is:
Santamaria defines it as:
All complex living beings, including single-celled resting phases alternate with periods of activity. As a more genetic complexity, the structure of sleep stages become more diverse and complete. The regulation of sleep-wake cycle is mainly controlled by the suprachiasmatic nucleus in the hypothalamus, which provides a daily rhythm called circadian. It is also regulated by a homeostatic instinct, that makes us to feel the need to sleep, perceived as drowsiness, after a long and continuous vigil. The mechanisms that control it in its extension are unknown, but there are conditions such as age, environmental factors and the emotional state of the individual. The quantity and quality of sleep needed in humans varies quiet significantly from individual to individual and depends on biological, behavioral and environmental factors.