Could you tell me what happens when you have a concussion and then go to sleep?

Depends: If the trauma that caused you to sustain a concusion also caused a slow bleeding into your brain or (sub dural area of your brain), than it could progress to something very serious. If you fell asleep, there would be no way of knowing if you were getting more disoriented or unresponsive, so it usually recommended to saty awake and be checked on regularly within the (at least)first 24h after.