Three vitamins are involved in regulation of the circadian rhythm.

Photoreceptors in the eyes of mammals synchronize the innate circadian rhythm to the solar light-dark cycle. They differ from the photoreceptors serving vision in rods and cones of the retina and are located in the ganglion cells of the retina. They consist of melanopsin, a protein homologous to rhodopsin, with retinal as the light-sensitive prosthetic group; and cryptochromes 1 and 2, proteins combined with methenyltetrahydrofolate and flavin adenine dinucleotide, which function as blue light-sensitive photoreceptors.