The Journal of Neurosurgery published an article in 2010 (Katayama, Becker, Tamura & Hovda) observing the chemical effects post brain trauma. Observing brain injury in rats, the study showed a release of Glutamate, an excitatory (stimulating) neurotransmitter. It is believed that this glutamate release coincides with the increase in potassium concentration immediately after injury. What is the significance of this? More excitation in an area means more demand for energy. This increase energy demand and excitability occurs at the same time as blood flow and oxygen is reduced in your brain. One study published in the American College of Sports Medicine's journal Medicine & Science in Sports & Exercise showed that a decrease in brain blood flow occurs under the stress of exercise for 3-7 days post concussion (Dec 2011).
More simply? Increased demand and decreased supply. If we were discussing material goods, this imbalance would increase the cost of the item. This also occurs in the brain. The cost is high. And this imbalance of supply and demand for oxygen and energy is what leads to the fatigue and "crash" post trauma. Using any part of our brain requires "fuel" and the more complex whether physical or cognitive, the more the brain will be taxed.
Can we affect demand? Increased cognitive challenge and then increasing physical challenge is adding to the already increased demand of the brain. If the individual has symptoms with increased cognitive challenge however, they should not even consider returning to sport or activity. The brain requires time to heal, but it is also important to add challenge within the means of the individual (without fatigue, brain fog or other symptoms).
Can we affect supply? Concussions are not only a physical issue, they are a chemical issue also. Maintaining a level blood sugar balance is key, and reducing the highs and lows we can get with refined sugar, or skipping meals will help support the brain during its most vulnerable time. Healthy fats and higher protein will also help to level out sugar levels. Again, this should also improve with time.
Most important to remember is that any injury is a full body experience therefore healing requires attention to the whole body. How we move matters. What we eat matters. What we think matters. The symptoms we experience with any injury are not the problem, they are the effects. Find the cause, you find the "cure".