Unless of course, you consume too few carbohydrates. The body will then have no choice about which fuel to burn for energy. The body doesn't become more efficient at burning fat and protein however, it simply has no choice because it is being carbohydrate starved. And this is NOT what leads to the effecient method of burning of body fat. There is another Zone Diet argument that carbohydrates make the body store fat by producing too much blood sugar.
Truth is, the body only converts refined material into too much sugar.
The body absorbs unrefined sugar from the consumed food we eat, then efficiently stores it as long as you are not over eating anything. If you consume most of your carbs as simple sugars, with negligible fat or quality protein, your blood sugar will peak a bit and then that sleepy feeling occurs as the pancreas secretes insulin to try and correct the misjustice of eating sugary foods. The sleepy feeling is not from lowered blood sugar. It is from the blood supply being diverted to your intestines to fix what you broke. You are merely playing games with the body's natural processes.