What happens as a result of sleep that causes the coma after a concussion?

Sleep is no problem: People lose consciousness as a result of concussion. It is not the apparent sleep, but what is happening to the brain trigging loss of consciousness. The concept of keeping someone awake is to monitor the course of brain injury. If the person falls asleep they can be awakened periodically to monitor their course. If they cannot be aroused, the injury is progressing needing intervention.