Biological rhythms are set by an internal clock, or pacemaker. These rhythms persist even in the absence of external cues. Many animals show rhythms in activity and physiological measures that repeat each day, month, or year. The internal clock that drives a circadian (daily) rhythm can be synchronized to time cues in the environment, such as the light/dark cycle. This process of synchronization to an external stimulus is called entrainment.
Until recently, much of the research on biological rhythms has focused on identifying behaviors that are influenced by endogenous clocks, and on determining how these behaviors can be modified, or entrained, by environmental stimuli.
Much of the current excitement in this area of research centers around the discovery of several key genes involved in the production and maintenance of internal clocks. Studies in both fruit flies and mammals have uncovered genes that are activated or deactivated in a cyclical pattern. The products of these genes—a protein cascade—somehow work together to control the biological functions and behaviors that vary in response to this internal clock. Further analysis of these clock genes should allow us to determine how endogenous clocks communicate with the rest of the body to control the biological rhythms that influence our lives.
Textbook Reference: Concept 29.6 Animal Function Requires Control Mechanisms