CLOCK and BMAL1 regulate circadian gene expression by binding to E-box elements within the promoters of clock controlled genes (CCGs). Some of the CCGs are regulators of metabolic pathways (e.g. Pepck, Glut2, Hmgcr), while some can influence metabolite levels (e.g. Nampt, Alas1). Nutrient sensors AMPK and SIRT1 are regulated in a circadian manner, and they can modulate circadian rhythms and metabolism by post-translational modifications of key regulators.