Phasic expression of genes drives the physiological and behavioral manifestations of circadian rhythms at the organismal level. Remarkably, animal clocks are well conserved from insects to mammals, revealing an important role in basic animal models to understand the mechanistic basis of human circadian rhythms. The discoveries found using these models have facilitated the understanding of the mechanism behind circadian rhythms and their significance to biology and disease. For humans, the most prominent circadian rhythm is the 24 h rhythm in the sleep-wake cycle.įour widely divergent model systems that have been historically used to study the genetics underlying circadian rhythms: fruit flies ( Drosophila), fungi ( Neurospora), cyanobacteria and mouse ( 3). Lastly, the periodicity of rhythms is stable across a wide range of temperatures, a property referred to as “temperature compensation” ( 2). Thirdly, although circadian activity rhythms are derived from an endogenous clock, they can adjust to the exogenous signals such as light or heat. The second principle dictates that even if there are no exogenous cues present, periodical patterns are still shown ( 1) indicating that these rhythms result from an internal time-keeping system. First, these rhythms closely mirror the 24-hour solar day, hence the word “circadian”, which comes from the Latin words “circa” (close or about) and “dian” (day). Internal “clocks” evoke a set of anticipatory responses to changes in their environment, which we refer to as circadian rhythms.Ĭircadian rhythms have four unique properties. Time dependent changes in these parameters have evolved in order to allow plants and animals alike to maximize their fitness according to external cues. Almost all organisms organize their physiology, behavior and metabolism according to the 24-hour solar cycle.
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