[Physiologic rhythm of sleep and waking].

The sleep-wake cycle is one of the endogenous rhythm of the body that become entrained to the day/night cycle. Light serves as a Zeitgeber, a stimulus that entrains an endogenous rhythm to the circadian clock. A direct retinohypothalamic tract (RHT) conveyed information about light to the internal circadian pacemaker. The primary biological clock is located in the suprachiasmatic nucleus (SCN). The RHT may serve to couple the circadian system to the external light/dark cycle.