The circadian system, sleep, and the health/disease balance: a conceptual review

Abstract

The field of "circadian medicine" is a recent addition to chronobiology and sleep research efforts. It represents a logical step arising from the increasing insights into the circadian system and its interactions with life in urbanised societies; applying these insights to the health/disease balance at home and in the medical practice (outpatient) and clinic (inpatient). Despite its fast expansion and proliferating research efforts, circadian medicine lacks a formal framework to categorise the many observations describing interactions among the circadian system, sleep, and the health/disease balance. A good framework allows us to categorise observations and then assign them to one or more components with hypothesised interactions. Such assignments can lead to experiments that document causal (rather than correlational) relationships and move from describing observations to discovering mechanisms. This review details such a proposed formal framework for circadian medicine and will hopefully trigger discussion among our colleagues, so that the framework can be improved and expanded. As the basis of the framework for circadian medicine, we define "circadian health" and how it links to general health. We then define interactions among the circadian system, sleep, and the health/disease balance and put the framework into the context of the literature with examples from six domains of health/disease balance: fertility, cancer, immune system, mental health, cardiovascular, and metabolism.

Keywords: circadian; circadian medicine; conditional process model; health; humans.

Figure 1:

A graphic of our hypothesis that the circadian system affects the health/disease balance through interactions with all three of its components: aetiology, severity of the actual disease, and recovery (see text for details).

Figure 2:

Formal aspects of the roles of elements in a network and how they can affect each other.

Figure 3:

An example of application of the conceptual framework with the addition of specific components, including Light, Activity-Rest (or Wake-Sleep) and the Clinic (where diseases are diagnosed and treated). Most of the components of the framework interact not only directly but also via feedbacks (see text for details).