ABSTRACT
INTRODUCTION The circadian system plays an essential role in regulating the daily cycles of physiological and behavioral functions, such as core body temperature, hormonal rhythms of cortisol and melatonin secretion, and levels of sleepiness, alertness, and performance (1). Circadian rhythms are endogenously generated cellular, physiologic or behavioral cycles with a genetically encoded period of approximately 24 hours. In mammals, circadian rhythms are regulated by a central circadian clock located in the suprachiasmatic nucleus (SCN) of the anterior hypothalamus (2-4). The endogenous circadian period in humans has been shown to be approximately 24.2 hours (5). Because under normal conditions, the circadian system is synchronized or entrained to the 24-hour rotation of the earth, individuals with an endogenous period longer or shorter than 24 hours require a daily net advance or delay of the SCN-driven rhythm (6). Light, melatonin, and high-intensity physical activity have been shown to act as synchronizing agents for the circadian clock. However, light is the primary circadian synchronizing agent in humans (7).
