Ana Barion, MD uman beings have adapted to living in a 24-hour environment by eveloping an internal timing system that follows a circadian rhythm. The erm circadian is derived from the Latin word circa, which means about, nd die, which means day. If allowed to free-run in the absence of ime-indicating external stimuli (zeitgeber), this human endogenous hythm will have a periodicity that is slightly longer than 24 hours, at 24.2 ours. The central pacemaker of the human body generating circadian hythms is the suprachiasmatic nucleus (SCN) located in the anterior ypothalamus. This highly regulated circadian rhythm established by the CN is sustained at the molecular level by an autoregulatory feedback oop of circadian clock gene proteins. In addition to controlling the 4-hour cycle of sleep and wakefulness, the SCN also maintains other hysiological processes, such as temperature, cortisol, and melatonin evels, in a circadian rhythmicity. To function optimally in a 24-hour world, the SCN needs to synchronize tself to the external time. This is achieved by receiving information from variety of sources, such as physical activity, social activities, and the nvironmental light and dark cycle. Light is the strongest zeitgeber (from erman, for time giver). The SCN receives direct input from ganglion ells in the retina. These cells produce a pigment called melanopsin and re particularly sensitive to light from the blue spectrum. Exposure to ight will phase advance or phase delay the sleep-wake cycle, depending n the timing, intensity, and duration of light exposure. Similarly, elatonin is an important zeitgeber. Melatonin is produced by the pineal land and is secreted under the direct influence of the SCN. Exogenous elatonin taken in the afternoon or early evening can phase advance the leep-wake rhythms, and when taken in the morning, will lead to a phase elay of the circadian rhythms. Therefore, light and melatonin have pposite phase shifting effects.