The Human Circadian System in Normal and Disordered Sleep

The human circadian system regulates rhythmicity in the human body and establishes normal sleepand wake phases. The suprachiasmatic nuclei (SCN), located in the hypothalamus above the optic chiasm,make up the human pacemaker known as the circadian or biological clock, but other essentialparts of the circadian system include the pineal gland, retina, and retinohypothalamic tract. The importanceof light in resetting the intrinsic human circadian cycle, the intrinsic period of which isslightly longer than 24 hours, ensures that the human cycle will stay entrained to the earth’s 24-hourdaily cycle. Within the SCN neurons, circadian rhythmicity is generated by the regular transcriptionof proteins. Since the circadian system is the foundation of the sleep-wake cycle, disorders and abnormalitiesin sleep are often connected with disorders or abnormalities in the circadian system. Circadianrhythm sleep disorders, such as jet lag syndrome and shift work sleep disorder, are those specificallyattributed to dysfunctions or insufficiencies in the circadian system. Taking into considerationthe preeminence of the circadian clock in timing sleep, it is likely that other sleep disorders, suchas insomnia, are also linked to circadian system abnormalities.