Photoperiodism is a biological response to light levels which occurs in many organisms around the world. Both plants and animals demonstrate this trait to varying degrees, and knowledge of how organisms respond to changes in light levels can be used to manipulate those organisms to produce particular desired outcomes. Researchers are also interested in photoperiodism and its potential applications.
Among plants, photoperiodism can alert a plant to the change of the seasons, which may trigger a variety of responses. For example, when the days are getting longer, the tree might start to produce buds and blooms, since it would sense that spring is on the way. As days shorten, the plant would start to become dormant, producing seeds for next year and taking steps to ensure that it would be ready for winter. Photoperiodism can also play a role in the setting of fruit and the movement of plants which adjust their position to take advantage of available light.
Knowledge of photoperiodism in plants has allowed botanists to trick plants grown in greenhouses by manipulating light levels to trick the plants. This technique is used to make blooms available year round for the floriculture industry, for example, or to force plants to produce seeds which can be distributed for sale. Researchers may also play with light levels in the process of researching variations between plant species, with the goal of learning more about these species and how they live in the natural environment.
This trait is also present in many animals. Animals with coats of fur often grow additional fur in the winter and shed that fur in the summer, responding to changing light levels to start this process. Photoperiodism can also trigger nesting, estrus, and other events in the lives of animals. It may also be related to responses to other environmental cues, such as temperature, rainfall, and so forth, with the animal's body responding to a constellation of indicators that the season is shifting.
While humans may think that they are exempt from photoperiodism, some evidence seems to suggest that humans actually do respond to changing light levels. Seasonal affective disorder (SAD), a common psychological condition, is directly tied to changes in available light and day length. This condition can cause severe emotional distress which can vary from depression to mania, suggesting that many humans are more closely tied to the change of seasons than they might admit.