Circadian clocks are synchronized by environmental cues such as light. Photoreceptor-deficient Arabidopsis thaliana mutants were used to measure the effect of light fluence Fate on circadian period in plants. Phytochrome B is the primary high-intensity red Light photoreceptor for circadian control, and phytochrome A acts under Low-intensity red Light. Cryptochrome 1 and phytochrome A both act to transmit low-fluence blue Light to the clock. Cryptochrome 1 mediates high-intensity blue light signals for period Length control. The presence of cryptochromes in both plants and animals suggests that circadian input pathways have been conserved throughout evolution.