The ABC of Psychological Medicine:

“Fatigue can refer to a subjective symptom of malaise and aversion to activity or to objectively impaired performance. It has both physical and mental aspects. The symptom of fatigue is a poorly defined feeling, and careful inquiry is needed to clarify complaints of “fatigue,” “tiredness,” or “exhaustion” and to distinguish lack of energy from loss of motivation or sleepiness, which may be pointers to specific diagnoses… ” A review of the concept and medical implications, in the British Medical Journal Sharpe and Wilks 325 (7362): 480