Biological Rhythms Health Article

Definition

A biological rhythm is one or more biological events or functions that reoccur in time in a repeated order and with a repeated interval between occurrences.

Description

Biological rhythms are the ways that organisms adapt and live with the environmental rhythms around them, such as the spin of the earth, the movement of the earth around the sun, and movement of the moon around the earth. Often generated by "biological clocks" (the term for the internal physiological systems that track the environmental rhythms), biological rhythms allow an organism to harmonize successfully with its environment. Although biological rhythms have not been studied in every living thing, they have been found in every organism in which experiments were performed. Accordingly, scientists believe biological rhythms are ubiquitous.

Generally, there are two types of biological rhythms, exogenous and endogenous. Exogenous biological rhythms are driven directly by the environment or another external influence. Another term for this type of biological rhythm is a direct effect. An example of an exogenous biological rhythm is the hopping of sparrows on a perch when a light is turned on. Such rhythms are said to have a geophysical counterpart; in this case, the presence of light.

In contrast, endogenous biological rhythms are driven by internal biological clocks and are maintained even when environmental cues are removed. Some examples of endogenous biological rhythms are the wake-sleep cycle and the daily body temperature cycles. Sometimes it is difficult to determine whether the activity of an animal is due to a direct effect or that of an endogenous biological clock, because the two types of rhythms can mask each other.

True biological clocks have four important characteristics. First, the clock is endogenous, meaning it gives the organism an innate ability to maintain periods of a particular length between biological functions. Experiments in space, with animals completely isolated from earthbound geophysical input, have supported the innate nature of the clocks. Second, the clock is temperature independent—a very unusual situation in biology but an essential characteristic to avoid biological rhythms being governed by the weather. Third, biological clocks have the ability to be reset in order to maintain a relationship with environmental cues. Finally, biological clocks are an internal continuous monitor of the passage of time, allowing the organism to keep track of duration biologically.

Chronobiology, the study of biological rhythms, categorizes rhythms by the length of the cycle. The most studied type of biological rhythm are circadian rhythms, which fluctuate on a daily basis. Alertness, body temperature, and the circulating concentrations of growth hormone, cortisol, and postassium are all examples of physiological functions that run on a circadian basis. Infradian cycles last about a month or longer. Menstruation in the human adult female is an example of an infradian biological rhythm. Circannual cycles last about a year; over-winter hibernation as a common example. The shortest cycles are ultradian, where the cycles are less than 24 hours. Heart rate and breathing are two examples of ultradian biological rhythms.

Function

The function of biological clocks and the resulting biological rhythms involves two factors: the capacity of the biological clock to freerun (operate without external cues), and the ability of timing signals, known as Zeitgeber (German for "time-giver"), to synchronize the cycles to the environmental signals. Some common Zeitgebers include light, temperature, and social cues such as clocks, sound, or physical contact. A biological clock is said to be freerunning when these external cues are removed. Based on multi-day isolation experiments, the average freerun period for circadian rhythms in humans is 25 hours. Thus, if isolated from outside input, people tend to go to sleep one hour later each day, quickly becoming out of sync with the rest of the 24 hour-based human world.

Entrainment is the process of aligning a biological rhythm with an environmental stimulus. There are limits to the time periods that biological rhythms can be entrained. For circadian rhythms in most animals, 18 hours is the shortest period tolerated, with an upper limit of about 28-30 hours. If Zeitgebers are provided for shorter or longer intervals, the organism reverts back to freerunning. A good example of entrainment is the acquisition of the 24-hour wake-sleep schedule by human infants after birth. Newborn circadian rhythms freerun, significantly disrupting the sleeping patterns of their parents. However, as they mature and become responsive to Zeitgebers such as light and dark, infants gradually adopt the 24-hour schedule of adults.