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The supra-chiasmatic nucleus (SCN) is the primarily biological clock determining the<br />circadian rhythm. The neurons of the nucleus making this clock have inherent rhythm<br />and set in biological day and night. These periods usually corresponds to day/night, and<br />indirectly to sleep-wakefulness cycle, in most individuals. Retino-hypothalamic tract<br />carrying photic information from the retina provides the most important input to<br />maintain the inherent rhythm of the SCN. The rhythmic discharges from the SCN to<br />various neurons of the central nervous system, including pineal gland and<br />hypothalamus, translate into circadian rhythm characteristic of several hormones and<br />metabolites such as glucose. As a result there is a pattern of hormonal changes<br />occurring during cycle of sleep wakefulness. Most characteristic of these changes are<br />surge of melatonin with biological night, surge of growth hormone-releasing hormone<br />(GHRH)􀀁at onset of sleep and surge of corticotropin-􀀁releasing􀀁hormone􀀁(CRH)<br />during late part of the sleep. The cause and effect relationship of the hypothalamic<br />releasing hormones and their target hormones on various phases of sleep including<br />initial non rapid eye movement (NREM) phase at onset of sleep, and rapid eye<br />movement (REM) phase near awakening, is an upcoming research area. Sleep<br />electroencephalogram (EEG) determining the onset of NREM and REM sleep is an<br />important tool complimenting the studies assessing relationship between various<br />hormones and phases of sleep. The slow wave activity (SWA) corresponds to the<br />intensity of sleep at its onset during the biological night of an individual. Besides,<br />GHRH and CRH, several other peptide and steroid hormones such as growth<br />hormone (GH), its secretagogues, ghrelin, neuropeptide Y, estrogen and<br />dehydroepiandrosterone sulfate are associated or have the potential to change phases of<br />sleep including initial slow wave-NREM sleep.