The mammalian cell contains a molecular clock that contributes, within each organism, to circadian rhythms and variety of physiological and metabolic processes. The clock machinery is constituted by interwined transcriptional–translational feedback loops that, through the action of specific transcription factors, modulate the expression of clock-controlled genes. These oscillations in gene expression necessarily implicate events of chromatin remodeling on a relatively large, global scale, considering that as many 10% of cellular transcripts oscillate in a circadian manner. CLOCK, a transcription factor crucial for circadian function, has intrinsic histone acetyltransferase activity and operates within a large nuclear complex with other chromatin remodelers. CLOCK directs the cyclic acetylation of the histone H3 and of its own partner BMAL1. A search for the histone deacetylase (HDAC) that counterbalanced CLOCK activity revealed that SIRT1, a nicotinamide adenine dinucleotide (NAD+)-dependent HDAC, functions in a circadian manner. Importantly, SIRT1 is a regulator of several metabolic processes and was found to interact with CLOCK and to be recruited to circadian promoters in a cyclic manner. As many transcripts that oscillate in mammalian peripheral tissues encode proteins that have central roles in metabolic processes, these findings establish a functional and molecular link among energy balance, chromatin remodeling, and circadian physiology.