Sleep
Sleep is a recurrent natural state of loss of consciousness of the external world, accompanied by a gradual decrease in muscle tone, occurring at regular intervals and whose role is still little known. Sleep-wake alternation corresponds to one of the fundamental cycles in animals: the circadian rhythm. In the human being, sleep occupies about one third of life on average. Sleep is distinguished from unconsciousness by the preservation of reflexes and the capacity of the sleeping person to open his eyes and react to speech and touch. There is an organization of sleep and its three states. It is a question of circadian cycle for the alternation between waking and sleep. There is talk of ultradian cycle for the alternation between slow sleep and paradoxical sleep. Sleep depends on the ventrolateral preoptic nucleus. Triggered by the daily accumulation of adenosine, the VLPO sends to the stimulation centers the signal to stop the production of histamine and other substances that keep us awake.