The sleep-wake cycle displays a characteristic 24-hour periodicity, providing an opportunity to dissect the endogenous circadian clock through the study of aberrant behaviour. This article surveys the properties of circadian clocks, with emphasis on mammals. Information was obtained from searches of peer-reviewed literature in the PUBMED database. Features that are highlighted include the known molecular components of clocks, their entrainment by external time cues and the output pathways used by clocks to regulate metabolism and behaviour. A review of human circadian rhythm sleep disorders follows, including recent discoveries of their genetic basis. The article concludes with a discussion of future approaches to the study of human circadian biology and sleep-wake behaviour.
The sleep-wake cycle, with its characteristic intervals of activity alternating with restfulness that recur with a periodicity approximating the 24-hour day-night cycle, is the prototypical example of a behaviour that demonstrates a circadian rhythm. Circadian (from the Latin “circa diem” – “about a day”) rhythms are also discernible in physiological and biochemical properties of the human body, such as core body temperature and corticosteroid and melatonin levels.
This article is available only as a PDF. Please click on “Download PDF” on top to view the full article.