Sugar is a stimulant, it will increase your blood sugar levels drastically, and then as the body attempts to counteract the high levels you will have a slump of blood sugar. Your energy levels slump, you get headaches, cannot sleep, cannot wake up, sweat to much, lack concentration, get irratible etc etc..
These are all of course extremes and a bit of sugar with your breakfast wouldn't cause them, but you'd be surprised how much your energy levels are raised in the long run if you cut out sugar altogether (as well as other stimulants). The body never evolved to digest glucose (refined sugar), it's too potent an energy source. Eat more complex carbohydrates that are slow releasing and keep your energy levels even.