For decades, the circadian system of animals was viewed as a single or a few centralized structures driving all overt physiological circadian rhythms. Recent data have questioned this view and have revealed a much more complex and dynamic picture. We now know that circadian oscillators are also present in many different tissues and constitute part of a hierarchical circadian system. In this article, we describe this multi-oscillator system that constitutes the basis for the variety of physiological rhythms.

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