When you sleep at night, you're essentially fasting. Hence the word "breakfast" -- breaking your fast. When you wake up, your brain needs a fresh supply of blood sugar (or glucose). Even though a sugary breakfast will increase your blood sugar quickly, you'll also experience a crash a couple of hours later.
Whole-grain oatmeal (like all whole grains), on the other hand, absorbs slowly into the body. Instead of spiking, your blood sugar will rise at a slow and steady rate.
Studies show that children who ate oatmeal for breakfast scored up to 20 percent higher on tests than children who ate sugary cereal. This supports the theory that dips in blood sugar affect memory and concentration.