Influence of the Circadian System on Disease Severity

The severity of many diseases varies by time of day. For example, adverse cardiovascular incidents peak in the morning, asthma is often worse at night, and temporal lobe epileptic seizures are most prevalent in the afternoon. These patterns may be due to the day/night rhythm in environment and behavior, or endogenous circadian rhythms in physiology, or a combination of both. Furthermore, chronic misalignment between the endogenous circadian timing system and the behavioral cycles could be a cause of increased risk of diabetes, obesity, cardiovascular disease, and certain cancers in shift workers. This article describes the magnitude, relevance, and potential biologic basis of such daily changes in disease severity and of circadian/behavioral misalignment, and presents how these insights may help in the development of appropriate chronotherapy.