Introduction to Circadian Clocks

Figure 1. Evolution of circadian rhythms. Red, Blue -- Organisms in which a circadian clock has been characterized.

Circadian clocks (from the Latin circa diem, “about a day”) are a very ancient feature of life on earth, and probably originated in photosynthetic archaebacteria. Since these organisms have no organelles, clocks were probably important to temporally segregate the mutually nuisible processes of photosynthesis and respiration. Indeed, in modern cyanobacteria, strains with a clock period length that matched the environmental photoperiod easily outgrow strains with no clock or with an unmatched period length in competitive coculture conditions (Woefle et al., 2004). In keeping with their ancient evolutionary origin, all known biological clocks are cell-autonomous: they are capable of functioning independently without external input from the environment or from surrounding cells. In addition, they are temperature-compensated, so that their period length -- the time that it takes for one cycle -- is the same within physiological ranges of temperature fluctuation.


Woelfle, M. A., Ouyang, Y., Phanvijhitsiri, K., and Johnson, C. H. (2004). The adaptive value of circadian clocks: an experimental assessment in cyanobacteria. Curr Biol 14, 1481-1486.