An endocrinologist's guide to the clock.

CONTEXT: It has long been recognized that a "biological clock" residing in the suprachiasmatic nucleus controls circadian or daily variations in physiological processes. Old observations are now being revisited after the discovery of the cellular mechanism of timekeeping, the molecular clock, an autoregulatory feedback loop of transcription factors that cycles over a period of approximately 24 h. Its functioning or breakdown impinges upon the physiology and pathophysiology of numerous systems, including the endocrine system and metabolism. Here we provide an introduction to those aspects of the clock most relevant to the endocrinologist. EVIDENCE ACQUISITION: Articles were identified by searching PubMed using the search terms "circadian" and "clock" and refining results to include articles relating to endocrinology and metabolism. EVIDENCE SYNTHESIS: We discuss current understanding of the mechanisms through which hormonal and metabolic axes fall under the influence of the circadian clock. Of particular interest is the complex interaction of genetic and environmental factors in determining health or disease states.
CONCLUSIONS: Research into the molecular clock provides a novel window onto endocrine and metabolic disease. These advances present new avenues for diagnostic and therapeutic strategies.