The human circadian system regulates rhythmicity in the human body and establishes normal sleep and 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 essential parts of the circadian system include the pineal gland, retina, and retinohypothalamic tract. The importance of light in resetting the intrinsic human circadian cycle, the intrinsic period of which is slightly longer than 24 hours, ensures that the human cycle will stay entrained to the earth's 24-hour daily cycle. Within the SCN neurons, circadian rhythmicity is generated by the regular transcription of proteins. Since the circadian system is the foundation of the sleep-wake cycle, disorders and abnormalities in sleep are often connected with disorders or abnormalities in the circadian system. Circadian rhythm sleep disorders, such as jet lag syndrome and shift work sleep disorder, are those specifically attributed to dysfunctions or insufficiencies in the circadian system. Taking into consideration the preeminence of the circadian clock in timing sleep, it is likely that other sleep disorders, such as insomnia, are also linked to circadian system abnormalities.