Costa R, Kyriacou C
Front Physiol
Circadian rhythms are ubiquitous and are observed in nearly all organisms that have been studied which inhabit our planet. These include animals, plants, fungi as well as some bacteria. These endogenous clocks will tick along nicely with an approximately 24 h period in constant conditions because they are generated by the so called “transcriptional-translation-feedback-loops” (TTFLs) that were first described in Drosophila. The pioneers of this fly research, Jeff Hall, Michael Rosbash, and Mike Young were awarded the 2017 Nobel Prize in Medicine or Physiology reflecting the importance of their contribution to understanding a fundamental feature of life, namely, rhythmicity. These TTFLs appear to be bolted on and interconnected to a more primitive metabolic oscillator, which, under certain conditions, can be observed in the absence of the TTFL in a number of model species (Edgar et al., 2012). While these oscillators are endogenous, their major role is presumed to be in anticipating the regular fluctuations in the environment that are associated with the Earth spinning on its axis every 24 h and preparing the organism for its daily chores. For animals this might mean foraging, finding a mate, outwitting predators and avoiding extreme daily heat or cold. In addition there are other geophysical cycles to which organisms are sensitive, for example the seasonal changes caused by the Earth’s tilt around its axis as it circles the Sun. There is also the gravitational pull of the Moon on the oceans which generates 12.4 h cycles in the ebb and flow of the tides to which shoreline algae, animals and plants have responded by evolving circatidal cycles of behaviour and physiology. Other lunar-related cycles include semi-lunar (∼15 days) and lunar cycles (29.5 days) which have important implications for the reproductive cycles of many organisms. All of these cycles, circadian and non-circadian entrain to environmental “Zeitgebers” (time-givers) the most important of which is the light cycle, but temperature cycles, social stimuli, seasonal photoperiodic changes, vibration or water pressure changes can be equally effective in “entraining” a biological clock to its optimal phase. It is entrainment that determines the time of day or chronotype, yet this property of circadian clocks is less well understood than free running rhythm.