My neurologist described it this way. Because part of my brain was injured, my brain is busy making new connections to go around the injured area. That burns a lot of calories and makes us tired. For me the concussion also triggered some sleep issues including sleep apnea, which meant I wasn't getting good sleep. I am sleeping better now and find I am less fatigued although I do have those moments from time to time when my brain shuts down. I've stopped trying to push through it if I can b/c it just makes the recovery time worse. Right now I am headed for a nap.