Actually I believe its due to an overstimulated insulin release. Insulin is the hormone that triggers the storage of blood suger storage. If you at a meal high in carbs expecially if they are high on the glycemic index, you can inadvertantly cause an overrelease of insulin. This is why many say high carb diets are bad, caue the insuline will force the glycogen out of the blood and store it, if your muscles dont need it, it gets stored as fat. At the same time, your brain runs on glycogen so if you have excess amounts of insuline storing glycogen you get sleepy cause your brian has no energy.