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Abstract: Circadian rhythms are widespread in nature and coordinate internal physiologic functions with external daily light-dark cycles. Key features of circadian rhythms include their persistence in the absence of external signals, their regulation by metabolic stimuli, and their ability to robustly compensate, and even over-compensate, for changes in external temperature. Multisite phosphorylation of Period (PER) proteins is a critical regulator of circadian period. I will discuss our recent phosphoswitch model, where two competing phosphorylation sites determine whether PER2 has a fast or slow degradation rate. This mathematical model accurately reproduces the complex degradation kinetics of endogenous PER2 observed experimentally. The phosphoswitch provides a biochemical mechanism for temperature compensation of circadian period. This phosphoswitch additionally explains the phenotype of Familial Advanced Sleep Phase (FASP) and CK1e^tau genetic circadian rhythms disorders, metabolic control of PER2 stability, and how drugs that inhibit CK1 alter period. The phosphoswitch provides a general mechanism to integrate diverse stimuli to regulate circadian period.