I know that the fatigue is not because of exertion or some other medical condition. It's essentially unexplained.
I am not entirely familiar, but I know it is not due to what usually causes fatigue which is ongoing exertion.
Do you know what Chronic fatigue syndrome is?
The fatigue is not due to strenuous ongoing exertion, is not much relieved by rest, and is not due to a previous medical condition. Fatigue is a common symptom in many illnesses, but the unexplained fatigue and severity of functional impairment in CFS is comparatively rare.