Chronobiology

Abstract

Sleep-activity cycles in animals and man, daily movements in plant leaves, and seasonal cycles of flowering, growth, and reproduction have been observed and noted since antiquity. The first experimental evidence that such biological rhythms were endogenous, and not simply driven by environmental cycles, was provided in 1729 by a French geologist, Jean Ortous deMairan, in a report to “1’Academie Royale des Sciences.” DeMarian observed that Mimosa plants retained their daily leaf movement pattern even when removed from their natural environment and transported to a continually dark cellar. Although some investigators later embraced a purely exogenous explanation for the generation of biological rhythms (the so-called “factor X” theories), by the late 1950s there was general agreement about the endogenous origins for near 24-h biological periodici-ties. Perhaps the single most compelling experimental result demonstrating the inherently endogenous nature of these rhythms was the observation that daily rhythms were not really “daily” when plants or animals were placed under constant environmental conditions. Rather, they deviated from the solar day; they were circadian, circa (about), diem (a day), consistently starting a little earlier or later than the day before. Moreover, different organisms measured at the same solar time, under the same experimental conditions, exhibited rhythms with periodicities that differed from one another. This observation rendered any purely exogenous explanation highly improbable.