Circadian Rhythm Sleep-Wake Disorders

The circadian system regulates a range of behavioural, physiological and cellular rhythms that allow organisms to anticipate changes in their physical environment, such as cycles of day and night. These predictive near-24-h oscillations persist in the absence of all environmental cues and are driven by the endogenous master pacemaker located in the suprachiasmatic nucleus (SCN) of the anterior hypothalamus in mammals (Stephan and Zucker 1972; Ralph et al. 1990; Edgar et al. 1993). Within individual SCN neurons, cyclic core clock genes and proteins establish a molecular basis of circadian control through transcriptional, translational and post-translational feedback loops (Reppert and Weaver 2002). This intracellular clock is not only found in the SCN, but in nearly all peripheral tissues of the mammalian body where it establishes tissue-specific gene expression to temporally coordinate physiology. Signals from the central SCN are relayed throughout the brain and body to synchronize these peripheral clocks to appropriate phases through neuronal, hormonal and physiological mechanisms (Buhr and Takahashi 2013). In this manner, the SCN regulates many diverse processes, including daily patterns of sleep, endocrine secretion, glucose homeostasis and core body temperature.