The light phase in a cycle of alternating periods of light and dark. Changes in light-dark cycles, such as changes in daylength, are the most dependable external time cues (zeitgebers). The physiological response to the length of the day or night, known as photoperiodism, is mediated by an internal or biological clock that synchronizes daily activities with external light-dark cycles. Most species studied show a circadian or daily rhythm set to a 24-hour cycle. Organisms also show long-term photoperiodism where their seasonal activities, such as courtship, mating, reproduction, migration, flowering, and seed production, are coordinated with others in the population and within the community.