Circadian clocks synchronize diverse biological processes in organism to 24-hour light-dark cycles. The rhythmic activation of selective pathways enables the organisms to optimize their ability to store and generate chemical energy, to minimize environmental stresses, and to reproduce through cell growth and division cycles. In mammalian tissues, major metabolic pathways exhibit robust diurnal rhythms, including glucose and lipid metabolism as well as mitochondrial fuel oxidation. This temporal organization of energy metabolism in relation to circadian timing adds a new dimension to the mechanisms that maintain energy homeostasis. Our recent study demonstrated that the transcriptional coactivator PGC-1alpha is a key factor that integrates clock and metabolic pathways. Here we discuss the implication of these findings in the context of molecular mechanisms that control metabolic oscillations in mammals.