the functional or behavioral response of an organism to changes of duration in daily, seasonal, or yearly cycles of light and darkness. Photoperiodic reactions can be reasonably predicted, but temperature, nutrition, and other environmental factors also modify an organism's response.

In animals, the regular activities of migration, reproduction, and the changing of coats or plumage can be induced out of season by artificially altering daylight. Birds, for example, have migrated north in the winter after having been exposed to reversed seasonal lighting in laboratories. The manipulation of a specific stimulating period of darkness, which is required by each species for every phase of the migratory process, is an important factor in photoperiodism.

When stimulated by light, an animal's pituitary gland will release hormones that affect reproduction. Thus, the mating season of a species can be made to occur at an unusual time by manipulating daylight. Long periods of light followed by short periods will induce mating behaviour in species that normally breed in autumn (e.g., goats and sheep), while spring breeders (e.g., mink) will start the reproductive process when daylight is increased. Application of photoperiodism is common in the poultry industry, as daylight affects egg-laying, mating, and body weight of the fowl.