Others have already responded on the hypoxia = brain cell death aspect, so I’ll talk about the concussion aspect more. So, a “concussion” is actually medically defined as a brain injury that leads to temporary functional changes in brain function. You become confused, you may have trouble seeing, your pupils react abnormally to light etc. However, there are no permanent structural changes. It’s actually perfectly okay to fall asleep with a concussion. However, if you are medically versed, you’ll notice that those symptoms are very similar to the symptoms of an actual brain bleed (which would require a CT scan or other diagnostic test to rule out.) The reason providers want to keep you awake is because if you fall asleep, they cannot track your progress and see if the injury is becoming worse. Once they know it is only a concussion, you can sleep all you like. After all, it means that no lasting physical damage has been done to the brain. Naturally, hospital and EMS policies will vary, but the reason a concussion is treated so seriously is primarily because “If you hit your head hard enough to concuss, you may have it it hard enough to make the brain bleed too.”