The ballance between endogenous and exogenous factors is remarkably relevant for biological rhythms, and it can be impaired by environmental changes and consequently provoke a series of disturbances such as sleep disorders, indisposition, gastro-intestinal changes 1 , mood fluctuations, irritability, tension, confusion, anxiety and reduced performance in tasks that require attention and concentration 2 . The sleep-wakefulness cycle is a circadian rhythm that, under natural conditions oscillates over a 24-hour period. The interchange of day-night (lightdark), school hours, work hours, leisure time, family activities, are exogenous factors that synchronize sleep-wakefulness 3 . Besides this environmental synchronization, the sleep-wakefulness cycle originates and is regulated endogenously by a neural structure located in the hypothalamus, it is the supraquiasmatic nucleus, considered the master circadian biological clock for mamals 4,5 . Optional hours, that prevent us from sleeping at habitual hours, considerably affect psychophysiological balance. The clinical adverse effects of the interruption of circadian rhythms, such as night-shift work include severe impairment during wakefulness 1 . Sleep is an important factor of synchronization between internal fluctuations and environmental cycles. In humans, the best example of organism and environmental synchronization is the sleep/ wake cycle, regulated by light and dark. According to their nature and social organization, human are active during the day, their physical functions are es-
NIGHTLY USE OF COMPUTER BY ADOLESCENTSIts effect on quality of sleep