Zeitgebers (time cues) for biological clocks

Abstract

The spatial and temporal aspects of the geophysical environment act as prominent selection forces for the evolution of life on this planet. The spatial features of the environment open up a choice of spatial niches and the temporal aspects on the other hand provide opportunities for adopting different temporal niches. Hence, both the spatial and the temporal properties of the environment together enhance the possibility for living organisms to exploit a given ecological niche at a given time of the day. The temporal selection pressures of the geophysical environment are composed of a number of abiotic factors such as light/dark cycles, temperature cycles, humidity cycles, and a range of biotic factors such as inter-individual interactions, interactions with preys, predators and parasites. Although the study of temporal organization in living organisms is relatively a recent phenomenon in biology, we now have access to a fair amount of knowledge about it in a number organisms ranging from cyanobacteria to humans. In this review, we shall focus mainly on three core questions related to timekeeping in living organisms: How are circadian clocks made to oscillate at desired frequencies?; What are the geophysical cycles that fine-tune circadian clocks?; Why are circadian clocks circadian?