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