Many plants and animals have the ability to measure changes in the duration of day and night and respond in ways appropriate to climatic cycles accompanying these changes. It was first thought that organisms with this capacity were measuring photoperiod (day length) and thus the phenomenon was called photoperiodism. The name persists, although it is now known that some organisms are sensitive to the duration of the dark or night period as well.

In animals, photoperiodism influences many seasonal activities such as migration, reproduction, and changes in plumage. Changing periods of daylight affect the animal's pituitary gland and cause it to release hormones that affect reproduction. By artificially manipulating day length, the mating season of a species can be changed. Thus, species that normally breed in the fall, such as goats and sheep, can be induced to breed at other times by exposing them to long periods of...