Biological rhythms, endothelial health and cardiovascular disease.

The activity of several components of the vascular system appears to be diurnally regulated. Endothelial cell activation, leukocyte and platelet interactions and lipoprotein metabolism have all been shown to vary with time of day, but whether these variations are due to the endogenous circadian clock, exogenous factors, such as the light-dark cycle, or an interaction between the two remains to be determined. Endothelium-dependent vasodilation also varies diurnally. This rhythmicity is lost in individuals with established coronary disease has been shown to occur in the early stages of atherosclerosis. The incidence of coronary events appears to be higher in the early hours of the morning, this may be related to heightened activity of the autonomic nervous system at this time. Higher circulating levels of catecholamines in the morning are associated with increased vascular tone, affecting circulating blood volume and blood pressure. Time dependent variations may be of particular significance for individuals with disrupted circadian rhythms, including rotating shift workers, transmeridian travellers and blind individuals with no light perception. Variations in endothelial function are observed during the menstrual cycle, varying with circulating oestrogen levels. Oestrogen deficiency in postmenopausal women may contribute to endothelial dysfunction, together with other modifiable risk factors. The absolute risk of coronary disease is greater for men than for pre-menopausal women. Following the menopause gender differences in coronary risk are thought to diminish, although this remains controversial. This review focuses on the influence of both endogenous biological rhythms and environmental factors on the function and health of the human vascular system.