Circadian rhythms are generated by well-conserved interlocked transcriptional feedback loops that involve clock genes. A growing body of research suggests that there is biological crosstalk between circadian rhythm and metabolism. We recently found that the key NAD biosynthetic enzyme, nicotinamide phosphoribosyltransferase (NAMPT), and NAD-dependent deacetylase SIRT1 play a critical role in such crosstalk, by creating a novel feedback loop in which NAD functions as a "metabolic oscillator". This review aims to summarize recent progress in elucidating the pathophysiological importance of clock genes, NAMPT-mediated NAD biosynthesis, and SIRT1, in the crosstalk between circadian rhythm and metabolism.