Role of interleukin-6 in stress, sleep, and fatigue.

Chronic low-grade inflammation, in particular increased concentrations of proinflammatory cytokines such as interleukin (IL)-6 in the circulation, is observed with increasing age, but it is also as a consequence of various medical and psychological conditions, as well as life-style choices. Since molecules such as IL-6 have pleiotropic effects, consequences are wide ranging. This short review summarizes the evidence showing how IL-6 elevations in the context of inflammatory disease affect the organism, with a focus on sleep-related symptoms and fatigue; and conversely, how alterations in sleep duration and quality stimulate increased concentrations of IL-6 in the circulation. Research showing that acute as well as chronic psychological stress also increase concentrations of IL-6 supports the notion of a close link between an organism's response to physiological and psychological perturbations. The findings summarized here further underscore the particular importance of IL-6 as a messenger molecule that connects peripheral regulatory processes with the CNS.