Literature from medicine focuses on the aetiology and relationship of fatigue to other physiological variables. The chronic nature of this type of fatigue is problematic when approached from a biomedical perspective tailored to acute care. Its non-specific nature and elusive aetiology make it a diagnostic challenge (Sharpe & Wilks 2002), and its subjective nature makes measurement difficult, although this has been facilitated by the introduction of psychometrically sound fatigue instruments (Temel et al. 2006, Hewlett et al. 2007). Measurement of fatigue is particularly important in chronic conditions, where it is critical to the testing of interventions. In the absence of identifiable physical disease, fatigue is assumed to have a psychological origin. Unex- plained fatigue is labelled ‘idiopathic’ or ‘unexplained’ chronic fatigue (Schmaling et al. 2003).