THE NEUROBIOLOGY OF CIRCADIAN RHYTHMS

Abstract

Daily rhythms in nature, such as the opening and closing of flowers or our patterns of sleep and wakefulness and their association with the perpetual alteration of night and day, were recognized in antiquity although their origins were not questioned until the eighteenth century. The French Astronomer Jean-Jacques d’Ortous de Mairan conducted an investigation into whether the leaves of the Mimosa plant opened in response to light.1 While de Mairan’s experiments were the first to question the origin of such daily rhythms, Augustin Pyramus de Candolle is credited with the first suggestion that they arose through an internal timekeeping mechanism. In 1832, de Candolle concluded that the rhythm of Mimosa leaf folding and unfolding observed under constant light conditions must come “from within the plant”,2 and because rhythms observed under such conditions express a period of only approximately 24 hours, they have come to be called “circadian” rhythms, from the Latin circa “about” and dies “day”. Almost a half a century later Charles Darwin came to a similar conclusion regarding leaf movements, writing further that “we may conclude that the periodicity of their movements is to a certain extent inherited”.3 As time progressed, investigators became increasingly aware that not only plants, but all organisms including humans displayed daily rhythms that were generated by an internal time-keeping system or endogenous biological clock.4-