Illustration of the KaiC circadian clock protein from the cyanobacterium Synechococcus elongatus in its nighttime state. Circadian rhythms are biological clocks with a period close to 24 hours in duration that help regulate patterns of physiological functions such as sleep and feeding in animals. Cyanobacteria are the simplest organisms that exhibit circadian rhythms. Oscillation of KaiC between the daytime and nighttime states is driven by a cycle of phosphorylation (night) and dephosphorylation (day).