Time is of the essence: vascular implications of the circadian clock.

The circadian clock is a molecular network of genes and proteins that control biological timing. In the suprachiasmatic nucleus of the brain, this unique signaling pathway operates to control the 24-hour sleep-wake cycle. In the periphery, the circadian clock acts to regulate local functions that follow circadian rhythms. Indeed, the cardiovascular system of mammals follows a circadian rhythm, exhibiting a functional oscillation of a 24-hour period. Blood pressure follows a 24-hour profile, rising and falling during the circadian day. Heart rate, endothelial function, circulating levels of humoral signals, and myogenic tone also follow a circadian rhythm. Recent data have demonstrated that the circadian clock resides in blood vessels; further evidence is defining the significance of the circadian clock in vascular cell signaling, blood pressure control, vascular function, and disease.