Why do people get sleepy after gettinga concussion and why do doctors generally tell you to not go to sleep after a concussion?

Brain swelling. Uncontrollable sleepiness/ grogginess, along with severe nausea are signs that the brain may be swelling or that there may be a bleed within the brain. It is important to frequently assess the injured person for such changes (which requires waking them) in the first 12 hours or so after a head injury since they would signal a need for medical re-evaluation and possibly even surgical intervention.
Brain stem injury. Fatigue after a concussion comes from a stretch injury to the nerves of the reticular activation system. The RAS is responsible for alertness and activating areas of the brain. In general, if symptoms of progression after concussion to a dangerous point will occur within 4-6 hours. If you do not experience a deterioration of mental status or alertness during that time go to sleep.