i asked this question to a dentist. it probably won't make much difference if you eat breakfast immediately after waking up, but it's actually best to brush your teeth as the first thing you do in the morning and before breakfast. you salivate less while you're asleep, and the time you spend asleep is ideal for bacteria to multiply and produce plaque. you'll benefit the most by getting rid of that plaque as soon as possible and before you introduce more food into your mouth for the bacteria to feed on.