19 Circadian Rhythms and Sleep

Abstract

Circadian (Latin for ~ one day) rhythms are cycles of behavior and physiology that occur with a periodicity of about 24 hours. The best-known example of such a rhythm is that of the sleep:wake or rest:activity cycle, but numerous other activities and physiological processes occur in a circadian fashion. Indeed, it is difficult to find an aspect of physiology that does not include such a rhythmic component, although many such rhythms are not relevant for the purposes of this chapter since their regulation is primarily non-neuronal. Rhythmic behaviors are controlled by endogenous clocks located within the nervous system, and these are the focus of this writing.

The importance of circadian rhythms is attested to by their ubiquitous nature, not only in the physiology and behavior of a specific organism, but also across the evolutionary tree. In humans, disrupted rhythms, as caused by shift work or travel to a different time zone (jet lag), are associated with serious health consequences. Genetic disorders that affect circadian function are also known and, in fact, have been mapped to circadian genes first identified in the fruit fly, Drosophila melanogaster (Toh et al. 2001; Xu et al. 2005). Clearly, these rhythms have evolved because they confer a selective advantage, the most obvious being adaptation to a cyclic environment. The presence of an endogenous approximately 24-hour cycle allows organisms to anticipate changes in the environment and to time their activities accordingly. In addition, it allows fine temporal control over daily functions/behaviors, such that processes which may...