Blue light-dependent regulator that is the input of the circadian feedback loop. Has no photolyase activity for cyclobutane pyrimidine dimers or 6-4 photoproducts. Regulation of expression by light suggests a role in photoreception for locomotor activity rhythms. Functions, together with per, as a transcriptional repressor required for the oscillation of peripheral circadian clocks and for the correct specification of clock cells. Genes directly activated by the transcription factors Clock (Clk) and cycle (cyc) are repressed by cry (By similarity). Binds 1 FAD per subunit. Interacts with tim and per; promoted by light conditions. Nuclear translocation initiates after the perception of a light signal. DNA photolyase domain is sufficient for light detection and phototransduction. FAD-binding region regulates cry stability, cry-tim interaction, and circadian photosensitivity. Belongs to the DNA photolyase class-1 family.