What is the term for the regulation of biological activity by day length?

The term that refers to the regulation of biological activity by day length is photoperiodism. This concept is essential in understanding how plants and animals respond to changes in the duration of light and darkness in their environment. Many organisms use the lengths of day and night as cues to regulate their physiological processes, such as flowering in plants or breeding in animals.

For example, certain plants may only flower when daylight exceeds a specific duration, while some animals may time their reproductive cycles according to the season's changing light. Photoperiodism helps synchronize biological activities with seasonal variations in climate, ensuring survival and reproductive success.

In contrast, seasonal variation broadly refers to changes that occur in biological organisms with the seasons, encompassing more than just light and day length. The diurnal cycle pertains specifically to the 24-hour cycle of day and night, influencing behaviors such as sleeping and feeding without focusing solely on longer-term seasonal changes. Chronobiology is the study of biological rhythms and how they are influenced by environmental factors, including light, but it is a broader field that includes various kinds of rhythms beyond just photoperiodism.