The Time Keeping System: a key organizational element

Abstract

Life has evolved in a cyclic environment, and consequently, during evolution the time dimension was incorporated in the inner organization of every single biological structure. This incorporation is evident in the biological rhythms detected in the whole phylogenetic range, from unicellular organisms to humans, and at all organizational level within an organism, from molecular to behavioral variables. ?Doing the right thing at the right time? enhances reproduction, health, survival and longevity. Biological clocks generate such rhythms, afferent pathways synchronize them with the cyclic environment, and efferent pathways convey this time information to the organism as a whole. In mammals, the main biological clocks are localized in the hypothalamus, the suprachiasmatic nuclei, which receive direct specialized inputs from the retinas allowing synchronization to the external night-day maintaining the organism in tune with its environment. A particular and refined set of photoreceptors allows this light perception. A network of peripheral clocks in other neural, as well as non-neural tissues throughout the organism, receives this time information assuring precise temporal relationship between every system within the organism, the so-called internal temporal order. When considering any biological system, time of sampling should always be tracked, and the use of basic and simple chronobiological methods considered so avoiding misinterpretations.