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The possession of a circadian timing system allows an organism to anticipate rhythmic phenomena. Rather than passively responding to 24. h changes in the environment, an internal clock allows an organism to anticipate these changes. Thus physiology and behavior can be fine-tuned in advance of the altered conditions and no time will be lost in the adjustment process. In mammals, the suprachiasmatic nuclei (SCN) of the hypothalamus are the site of a master circadian pacemaker, coordinating rhythms throughout the body, including sleep-wake cycles. The primary input into the SCN is light, and the light-dark cycle produced by the rotation of the Earth provides a robust signal enabling internal and external cycles to be synchronized (entrained). The influence of light upon this central clock is mediated by photoreceptors within the eye, but until recently it remained unclear which ocular cells provided dawn/dusk information for photoentrainment. Research into the mechanisms of mammalian photoentrainment resulted in the identification of a third class of ocular photoreceptor, quite different from the rods and cones. This system is composed of a subset of 'blue-light'-photosensitive retinal ganglion cells expressing the photopigment melanopsin. Subsequent studies have shown that these cells not only regulate the circadian system but also mediate a broad range of other irradiance-detection tasks, including pineal melatonin suppression and pupil constriction. © 2009 Elsevier Ltd All rights reserved.

Original publication




Journal article

Publication Date



669 - 676