Reconsidering mammalian circadian organization

A circadian clock enables an organism to occupy a particular temporal niche, and the evolutionary pressures associated with that temporal niche in turn shape the organization of the circadian network in which the clock is embedded. Although circadian organization has often been dichotomized into centralized and distributed systems, our modern understanding of circadian networks reveals a more complex organization that cannot be captured by this simple dichotomy. In this review, we examine how coupling between nodes of the circadian network (from cells, to tissues, to organs) gives rise to coherent circadian organization in mammals. We further highlight how comparative research on non-mammalian organisms reveals conserved and divergent strategies for circadian coupling that inform general principles of circadian network function across species. • Circadian clocks enable temporal niches; temporal niches shape circadian networks • Circadian coupling is the basis of all circadian organization • Non-mammalian clocks can reveal aspects of mammalian circadian network properties