Response by an animal or plant to changes in daily, seasonal, or yearly cycles of light and darkness. Among animals, sleep, migration, reproduction, and the changing of coats or plumage are regulated to some extent by day length. In the poultry industry, photoperiodism is commonly induced by artificial lighing to maximize egg laying and body weight. Plant growth, seed setting, germination, flowering, and fruiting are also affected by day length. Other environmental factors that modify an organism's responses include temperature and nutrition.