Introduction The accepted model for the molecular machinery that generates circadian rhythms involves a number of clock genes and their products. Clock genes function as transcriptional activators and regulate their own expression through a positive/negative feedback loop that cycles with a “free-running” period of approximately 24 hours. The genes CLOCK and BMAL1 form the positive regulator of the system while the genes PER 1–3, CRY 1–2, and REV-ERBα generate the negative feedback loop . In this way, they allow the development of circadian and seasonal rhythms in lower organisms and in humans, with allelic variants influencing individual rhythms at a behavioral and cellular level [2,3]. The control of circadian rhythms does not involve only clock genes, but it is a complex process implicating also environmental and physiological factors . In mammals the circadian clock shows a hierarchical organization. The primary circadian pacemaker, the master clock, is located in the suprachiasmatic nuclei (SCN) of the anterior hypothalamus  and is synchronized to external 24-hour light/dark cycles by photic inputs coming from the retina through the retino-hypothalamic tract . Light is a strong timing cue (zeitgeber), and the pacemaker's oscillation can be reset by light-induced phase shifts. The master pacemaker then communicates the timing information to peripheral oscillators present in the cells of most tissues. These signals include both direct (hormone secretion, sympathetic enervation) and indirect (body temperature, feeding intake) cues .
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