It's a side effect that is absolutely intrinsic to THC, you can't avoid this unless you have a chemical imbalance that makes the stuff affect you opposite of most people (i've seen this before, very odd to see a stoner with too much energy). Weed is a downer, its like trying to keep from getting hyper while under the effects of cocaine. Now I will say there are breeds of marijuana that are more of a "cerebral" high in that your body does not feel near as lazy as the basic stuff most of us get, but that's generally either genetically modified strains or just grown under extremely controlled conditions. In either case, you're gonna have a hard time finding it and you will definitely pay top dollar. Sometimes the pesticides that are used on mass-produced farms can have adverse side effects, like the paranoia you feel sometimes, and it probably helps in making you drowsy as well. But like I said, weed is a downer and it's going to slow you down regardless.