Introduction to Circadian Rhythms

Circadian rhythms have a period of approximately 24 hours and are set to precisely 24 hours by various zeitgebers (time givers), light being the most prominent zeitgeber. The central pacemakers for mammalian circadian rhythms are the suprachiasmatic nuclei (SCN) in the anterior hypothalamus. Humoral and neural signals from the SCN help synchronize circadian clocks throughout the body. At the molecular level, cellular circadian rhythms are formed from interlocking transcriptional-translational feedback loops (TTFL) of circadian clock genes that drive spontaneous oscillations of gene and protein expression with an approximately 24-hour period. Remarkably, the molecular clock components are expressed rhythmically in nearly every cell of the body and are entrained by signals from the SCN. Disruption of clock genes either through genes or environment can impair optimal biological function. Circadian rhythms regulate myriad homeostatic systems including the cardiac, immune, metabolic, and central nervous systems. Circadian regulation of physiological and behavioral functions can be disrupted by several factors including the timing of light exposure and food intake. This chapter reviews circadian disruptors to set up the remainder of the book.