Biological mechanisms of chronic fatigue.

Chronic fatigue is a common, poorly understood and disabling phenomenon in many diseases. We aim to provide an overview of fatigue in chronic autoimmune and inflammatory disease. Fatigue measurement, prevalence and confounding factors such as depression, sleep disorders and pain are reviewed in the first half of the article. In the second half of the article, we describe explanatory models of fatigue and fatigue signalling, with an emphasis on cytokines and sickness behaviour, oxidative stress, mitochondrial dysfunction and the impact of certain genes on fatigue.