Circadian Rhythms, Genetics of

As with the stress response, the expression of circadian rhythms enables an organism to adapt to changes in the external environment. Of course, the changes in the environment that represent the substrate for the evolution of circadian rhythmicity occur on a relatively predictable basis that coincides with the 24-h rhythms in the external environment due to the rotation of the earth on its axis. Interestingly, many of the metabolic and endocrine events associated with stress also show predictable circadian rhythmicity, indicating that these two adaptive systems are highly integrated with one another. Both systems are under tight genetic control, and this article focuses on (1) the genes underlying the generation of circadian rhythms, (2) the recent evidence that most, if not all, tissues of the body, including those involved in the stress response, contain the molecular circadian clock core machinery, and (3) the evidence that disruption of the molecular clock can lead to tissue and organismal dysfunction. © 2007