No, doctors still do not know the cause.  It could be biology, genetics or in someone's head from what I understand anyways.
Do you know what Chronic fatigue syndrome is? I know that the fatigue is not because of exertion or some other medical condition. It's essentially unexplained. Any idea what causes it? Ive heard of it, and of course have felt fatigue just from everyday life, but I cant say its chronic..
Biological, genetic, infectious, and psychological mechanisms have been proposed, but the cause is not understood.