The circadian clock: from molecules to behaviour.

Circadian rhythms are a cardinal feature of living organisms. The stereotypical organization of homeostatic, endocrine and behavioural variables around the 24-hour cycle constitutes one of the most conserved attributes among species. It is now well established that circadian rhythmicity is not a learned behaviour, but is genetically transmitted and therefore subject to genetic manipulations. Recent advances in the circadian field have demonstrated that circadian oscillations are cell autonomous, that the circadian mechanism operates through a negative feedback loop and that a growing number of genes is under circadian control. Furthermore, single-gene mutations have been isolated in mammals that have profound effects on circadian behaviour. The production and mapping of one of these mutations in the mouse, an organism about which there exists a wealth of genetic information, should accelerate the elucidation of the molecular events involved in the generation of circadian rhythms in mammals.