Biological Rhythms

Daily rhythms in plants and animals have been noticed since early times. As early as the fourth century B.C., Alexander the Great's scribe Androsthenes noted that the leaves of certain trees opened during the day and closed at night. Two centuries before modern gardeners noticed their day lilies closed at night, the famous taxonomist Carolus Linnaeus discovered the petals of many flower species opened and closed at regular times. He even created a garden with flowers that opened at various times so that he could tell the time of day by looking in his garden.

Karl von Frisch observed that bees visited flowers only at specific times. He and Ingeborg Beling trained bees to visit a nectar feeding station between 4 and 6 P.M. The bees did not visit at other times, and they still visited even when the nectar was removed. When outside cues such as light were removed in laboratory trials, the bees still fed at prescribed times. Although von Frisch did not know it, the bees were operating on an internal clock. It wasn't until the 1950s that Gustav Kramer and Klaus Hoffmann proved the existence of a biological clock. With an ingenious apparatus, Kramer demonstrated that starlings used the sun as a compass to migrate even though the sun itself moves throughout the day. That is, the bird's internal clock reorients it in the direction of the moving sun. Hoffmann showed that the clock persisted in dim light and thus is endogenous to the animal. He showed that the animal's clock was synchronized to local time by the influence of the local environment.

Research in the area of biological clocks did not begin with humans. Early research was done on a variety of animals including rats, hamsters, sparrows, lizards, marine snails, and fruit flies. The choice of an animal depended on many considerations—size, rate of reproduction, expense to maintain, and availability—as well as behavioral issues, such as whether it entrained easily and what environmental factors seemed to affect its behavior.

In 1920, a landmark paper was written by W.W. Garner and H.A. Allard in which they showed that tobacco plants would flower only if exposed to a certain number of hours of light. The term "photoperiodism" was used to designate the response of organisms to relative length of day and night. The ability to sense day length is an important ability for plants so that they grow, reproduce, and develop during favorable times of the year. The changing times of dawn and dusk contain seasonal information as well as time of day information so that the organisms have, in effect, an internal clock and calendar. In plants, a photoperiodic clock not only controls flowering, but also induction and termination of dormancy in buds and bulbs, seed germination, and daily rhythms such as leaf movements, petal movements, and nectar secretion.

The study of biological rhythms is called chronobiology, a relatively new scientific specialty that began in the 1950s when researchers used new heart and lung monitors to answer some of the basic questions that chronobiologists ask. Some of these questions deal with sleep/wake cycles, time of day energy and productivity levels, and mood changes. Other important areas of study in chronobiology deal with growth patterns, hormone secretions, and menstruation.