The reason people feel fatigued when they are sick or in pain is that the body is trying to force them to conserve energy for healing or fighting pathogens. It is not a direct effect of illness or injury, rather it is the result of hormones secreted into your bloodstream from the hypothalamus and pituitary gland in the brain when they sense high levels of inflammation (signifying damage to the body) in the bloodstream.

This means that the extreme fatigue we get after head injury is usually not a coincidence, or a symptom which we should try to overcome - it is the brain itself telling us that it needs us to rest.

Caffeine messes with this safety mechanism and makes us energetic when we would otherwise be feeling tired. The brain is telling you to rest and caffeine overrides that natural response, making you excited and energetic.

That's why I'm on the decaff too! If my brain is telling me to rest, I want to know about it.

I would add that in some more severe brain injuries fatigue might be caused by parts of the brain's arousal systems being damaged - in this case what I said before about it being a deliberate healing strategy would not apply, and caffeine may perhaps be useful.