Caffeine decreases cerebral blood flow. Alcohol increase cerebral blood flow. Since every brain injury is different, making a blanket statement of what caffeine does in each individual cases is difficult. If you improve without caffeine, that suggest that the negative effects of caffeine are an issue for you.

Brain injuries are known to cause increased sensitivity to various chemicals, caffeine, MSG, sugar, artificial sweeteners, etc. But, each person is different. For me, caffeine after 3 PM will effect me well past midnight. I might not be able to settle into sleep until 6 AM.

One rat study found caffeine in large doses to be beneficial in severe brain injuries during the immediate acute phase.

Overall, the consensus has been to avoid caffeine with a single serving being acceptable to start the day for some.