[Physiology of normal sleep].

Normal sleep of a young adult is composed of around 4 to 6 cycles of 90 minutes each. Two states of sleep have been established, REM sleep and Non REM sleep. REM sleep is mainly characterized by a complete atonia which creates a physiological paralysis. Slow wave sleep is characterized by an increase of more than 20% of EEG delta waves. Sleep could be represented by a two process model. The first one, the homeostatic component, represents the propensity to sleep, the second one, the circadian component, symbolizes 24th light-dark cycle of the rotation of the earth. The homeostatic component is probably linked to the accumulation of a metabolic molecule, adenosine which concentration increases progressively during the day in human brain tissue, especially in the hypothalamus facilitating the onset of sleep. The sleep-wake cycle is under the control of a biological clock located in the supra-chiasmatic nuclei (SCN) of the hypothalamus. The internal period of the SCN is roughly 24.5 hours setting to 24 hours by the light-obscurity cycle and the melatonin secretion by the pineal gland. The biological clock is regulated by complex mechanisms involving the production of specific proteins inhibiting their own genes by a negative feedback control.