If your breath stinks, it means you aren't eating enough carbohydrates to maintain proper brain function. Seriously. Diet after your exam, not before. Your brain needs carbs. It can't burn fat or protein, which is why your body starts producing ketones. Ketones, however, only provide about 2/3 of the necessary fuel for the brain. They put epileptics on low-carb diets deliberately to slow down the electrical activity in their brains.