Transcriptional regulation of circadian rhythms in mammals involves a negative-feedback loop whereby a CLOCK-BMAL1 complex activates transcription of the genes encoding the proteins PER1 (Period 1), PER2, CRY1 (Cryptochrome 1) and CRY2, which feed back to inhibit the activity of CLOCK-BMAL1. It is becoming evident that chromatin remodeling has a crucial role in the transcriptional regulation of genes governing circadian rhythms. This is highlighted in a recent report that shows that CLOCK has histone acetyltransferase activity, thus, demonstrating that molecular components of the biological clock can themselves catalyze chromatin modifications.