Abstract: Within this vast universe, the earth rotates on its axis, and also orbits around the sun, which creates rapid changes of day and night and the seasons of the year. For these reasons, the natural cues of light, temperature, humidity, nutrition and other environmental factors essential for individual survivals change periodically. The organisms have to adapt to the environment caused by the selective pressures to ensure their reproduction. Of all the fitness, the adaptation to light/dark cycles is most important. Creatures at different latitudes on the earth experience the rhythmicity of sunlight intensity, variable range of light quality and illumination length during the year, and they choose the suitable location to complete the growth, development and reproduction. Evolution has instated the timekeeping mechanism from circadian clock to confer organisms the ability to integrate and foresee the daily and seasonal changes in light exposure, and then improved survival rate by scheduling the physical and metabolic process at the most appropriate time. The studies concerned with the functional connection between biological clock and photoperiodism, and the more results shown that the circadian oscillators are involved in the regulation of plant’s or animal’s capacity to respond to the photoperiod.