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.

In 1729, French astronomer Jean Jacques d'Ortous deMairan conducted the first known experiment on biological rhythms. He noticed that his heliotrope plant's leaves opened during the day and folded at night. When he put the plant in total darkness, the rhythm continued, however, and was not disrupted by the absence of daylight as an environmental cue. Although he was interested in this botanical phenomenon, deMairan pursued astronomy instead.

Two centuries before modern gardeners noticed that their day lilies closed at night, the famous taxonomist Carolus Linnaeus discovered that the petals of many flower species opened and closed at regular times. He even created a garden with flowers which opened at various times so that he could tell the time of day by looking in his garden.

Nothing exciting happened for many years...

until the early 1900's, when 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 PM . 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 1950's 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.

In the 1950's, Colin Pittendrigh demonstrated that circadian clocks are temperature compensated (have nearly the same period even when the temperature changes). Most metabolic activities increase when body temperature increases, but the period of the biological clock does not. Pittendrigh placed Drosophila pseudoobscura cultures at different temperatures and recorded the time of eclosion. Even in constant darkness, the flies emerged on schedule regardless of the temperature. Temperature changes thus did not affect the period of the clock [1].

Reproduced, with permission, from the Annual Review of Physiology, Volume 55, Copy. 1993, by Annual Reviews Inc.