"Operating at maximum capacity induces fatigue, as we know when we compare how we feel after sitting a three-hour examination with sitting for three hours of idle chatting. After Mild Head Injury, activities (such as talking with visitors, for example) that do not normally demand full capacity may now use up all available processing space"

Dorothy Gronwall

That sums up perfectly exactly how PCS fatigue feels to me. I'm sure poor diet, depression and other such factors can contribute to making you tired, but the crippling fatigue that comes specifically with PCS is, in my opinion, definitely a result of the brain just having to work at full capacity the whole time to do even the simplest things.