Previous analyses of the circadian clock in the plant Arabidopsis thaliana have turned up just a few genes regulated by the clock. In the 15 December Science, Harmer et al. use oligonucleotide-based arrays to find a vast new collection of clock-regulated genes (Science 2000, 290:2110-2113). Using probes derived from tissue harvested every four hours, and an array representing 8,200 different genes, Harmer et al. find that 453 genes (6%) fit a cosine test wave with a period between 20 and 28 hours. The day begins with induction of photosynthetic genes, and genes for making photoprotective pigments. Genes for the energy-intensive process of nitrate assimilation are induced only during the daytime. As night approaches, genes for chilling resistance are induced, and metabolic and sugar transporter genes are induced, so that energy resources can be mobilized for the long night ahead. Energy management is further aided by...