Chronobiology in epidemiology and preventive medicine

Many of the known cardiovascular risk factors, such as hormones, metabolic parameters, lifestyle, blood pressure, fibrinogenesis, fibrinolytic activity, etc., and also the reactivity of the organism to stimuli, are periodical and therefore require adequate measures at specific times in appropriate frequencies. The presence of circadian, circannual or other wide frequency rhythms may lead to contradiction in the evaluation of a potential risk factor. Chronobiological analysis shows fine differences within a "risk variable", defining some characters which may imply the alteration of a parameter only at certain times. Periodical risk factors may therefore be foreseen and sometimes prevented. Moreover, chronobiological approach may show another peculiar relation between risk factors; this is the so-called chronorisk. The convergence, in certain periods of time, of the phases of various risk factors, may strengthen their combined value. On the other hand, the phases of the protective factors, in the same period of time, may contribute to reduce or strengthen this combined value. From this point of view combined risk, apparently unimportant, appears highly increased, as compared with the normal homeostatic value. At equal levels of risk factors, different distribution and amplitude of the phases implies great differences in final combined risk values. Acute heart attack has a daily, seasonal and perhaps ultradian rhythm; other cardiological events, such as angina pectoris and sudden death, have circadian rhythms. Also cerebral vascular events have daily, seasonal, weekly and monthly variations. The periodical variations of acute vascular events and of the related risk factors, near and far, implies many complex interrelations in timing of biological structures, which are still perceived by the physician only as "noise".