Stress-responsive genes are circadian regulated. (a) Circadian-regulated heat-induced genes are expressed before subjective dawn, completely out of phase with cold-induced genes. The average expression profile of heat-induced genes is indicated in red (n = 30), whereas that of cold-induced genes is indicated in blue (n = 46). (b) Circadian-regulated genes responsive to the reactive oxygen species hydrogen peroxide or to oxidative damage are expressed during the early subjective day. The average expression profile of genes induced by these compounds is shown in black (n = 41); for comparison, the average expression profile of genes involved in the light-harvesting reactions of photosynthesis is shown in orange (n = 60). The data shown are from the combined Covington plus Edwards data set CCEE. Mean expression levels are plotted on the y-axes and time in constant light is plotted on the x-axes. White and gray shading represent subjective day and night, respectively.