The importance of maintaining a smooth-running circadian clock becomes painfully evident whenever we suffer severe jet lag. Traveling through multiple time zones decouples our biological rhythms from the natural cycle of light and dark we’re used to. These light–dark cycles synchronize everything from basic metabolic processes to feeding behavior. People with total blindness, who cannot perceive these light cues because they lack functional retinal photoreceptors, have disrupted circadian rhythms. As mammals, we perceive light cues through retinal photoreceptors that relay the signals to a cluster of some 16,000 neurons in the hypothalamus called the suprachiasmatic nucleus (SCN).
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