The mammalian circadian clock is regulated by a negative feedback loop that comprises period (PER) and cryptochrome (CRY) protein complexes; they bind to the DNA-bound transcription factor CLOCK–BMAL1 to repress their own expression. In this study, Kim et al. show that mouse PER complexes contain several subunits of the NuRD transcriptional corepressor, whereas two other NuRD subunits, CHD4 and MTA2, associate with CLOCK–BMAL1. Knockdown of NuRD components showed that MTA2 and MBD2 promote PER-mediated transcriptional repression; surprisingly, CHD4 seems to promote CLOCK–BMAL1 transcriptional activity. The authors propose a model in which NuRD components are split between CLOCK–BMAL1 and the PER complex; the fully functional NuRD complex is reconstituted during the circadian negative feedback phase, when PER complexes target the remaining components to CLOCK–BMAL1, resulting in repression.