The near 24-hour oscillations of brain and body functions, such as the core body temperature, secretions of endogenous hormones, and the sleep-wake cycle, constitute the circadian rhythm. These rhythms are generated by intrinsic molecular mechanisms in the suprachiasmatic nucleus (SCN) neurons and are modulated by receiving input from light, activity, sleep-wake state, and other environmental cues. There is now clear evidence identifying the SCN as the master pacemaker regulating these rhythms. The SCN responds to environmental cues by sending outputs to various regions of the body to regulate the timing of behavior and activity, endocrine cycles, body temper ature, and the sleep-wake cycle. Also, dynamic interaction exists between the circadian rhythm and the homeostatic sleep drive that regulates the consolidation of sleep and wake performance. Disruption of this interaction occurs during shift work, jet lag, or other circadian sleep disorders, such as advanced or delayed sleep phase syndrome, and often leads to sleep-wake disruption.