RESUMEN
Human sleep, defined on the basis of electroencephalogram (EEG), electromyogram (EMG) and electrooculogram (EOG), is divided into rapid eye movement (REM) sleep and four stages of non–rapid eye movement (NREM) sleep. Collective monitoring and recording of physiological data during sleep is called polysomnography. Sleep which normally starts with a period of NREM alternates with REM, about 4-5 times, every night. Sleep pattern changes with increasing age. Newborns sleep for about 14-16 hours in a day of 24 hours. Although there is a wide variation among individuals, sleep of 7-8.5 hours is considered fully restorative in adults. Apart from restorative and recovery function, energy conservation could be one of the functions of sleep. The role of sleep in neurogenesis, memory consolidation and brain growth has been suggested. Though progress in medical science has vastly improved our understanding of sleep physiology, we still do not know all the functions of sleep.