Circadian Rhythms and the Transcriptional Feedback Loop (Nobel Lecture)**

Circadian rhythms are present in most if not all animals, plants, and even photosynthetic cyanobacteria. These cyanobacterial clocks as well as plant clocks are very different from those of animals, with no credible homology between the different clock proteins. Therefore circadian rhythms probably emerged multiple times in evolution, which underscores their importance.