Chronobiology: Implications of circadian rhythms

Biological rhythms are pervasive. All eukaryotes and at least one group of prokaryotes, the cyanobacteria, exhibit biological rhythms. Further, all levels of biological integration, such as ecosystem, population, group, individual, organ-system, organ, tissue, cell and subcellular structure show rhythms with diverse frequencies. Chronobiology (chronos - time; bios - life; logos-science) is the branch of science that deals with the study of biological rhythms and their mechanisms. It is multidisciplinary in nature and burrows techniques from many other disciplines. Circadian rhythms, popularly called biological clocks, are the most-studied mechanisms. They exhibit three important properties: 1) persist (or free-run) with a tau (tau) very close to 24 h in the absence of environmental time cues like day and night cycle; 2) entrain to 24-h cycles of environmental zeitgeber, most commonly the light/dark transition at dawn and dusk; and 3) possess temperature compensated free-running periods over a wide range of environmental temperatures, but within the physiological range. The basic circadian system consists of at least three interconnected components: 1) input pathways (photoreceptors, thermoreceptors etc.); 2) pacemaker (clock, such as SCN); and 3) effector pathways (overt rhythms). Molecular bases of cyanobacterial and eukaryotic circadian clocks have been partly understood with respect to their period length, sustainability, and relationship with light. The underlying molecular mechanisms of these clocks seem to have extraordinary degree of evolutionary conservation and appear to be a universal feature of the clocks in models as divergent as algae, fungi, fruit flies, mice and humans. This article outlines the concepts and current developments in the field of chronobiology with a reference to research on circadian rhythms. It also discusses the implications of circadian rhythms in the optimization of treatments in the clinics and in the management of problems in shift workers.