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.
Although the existence of biological rhythms has been apparent for quite some time, a detailed analysis of this phenomenon required the development of appropriate methods for measuring ongoing behavior over an extended period of time. This animation shows examples of several experiments that demonstrate how biological rhythms can be measured and how the existence of internal clocks can be revealed.
Most functions of any living system display a rhythm of approximately 24 hours. Since these rhythms last about a day, they are called circadian rhythms. Humans and most other primates are diurnal. That is, we are active during the day and sleep at night.
One way to study circadian rhythms in the laboratory takes advantage of the penchant of rodents to run in activity wheels. A switch attached to the wheel is attached to a microcomputer or chart recorder that registers each complete revolution of the wheel. The activity of sequential 24-hour periods can thereby be compared to determine the consistency of the circadian rhythm.
Like most other rodents, mice are nocturnal—active during periods of darkness. If we compare the activity record of the mouse over a period of a week, we see a fairly consistent record, with the mouse becoming active shortly after the start of the dark phase of the daily cycle.
To determine if the rhythm is produced by an endogenous clock, we can test the animal in an environment where external cues have been removed. For example, we can place the mouse in a constantly darkened environment.
Note that the duration of the rhythm in complete darkness is a little less than 24 hours, causing a leftward shift in the data record. Since this period does not quite match the period of Earth's rotation, it cannot simply be reflecting an external cue but must be generated inside the animal. A period that is independent of external cues is called a free-running period.
The free-running period is the animal's natural rhythm, and it suggests that the animal has some sort of internal oscillator, which we call a clock. In the mouse, this clock runs a bit fast.
The internal clock can be set by light. If we expose a free-running nocturnal animal to 20 minutes of light every 24 hours, the rest–activity cycle is entrained to a 24-hour period.
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.