It's not even a question of IF for those who are into this stuff. It's really a question of WHY for the researchers. But for those in the field, the solution is simple and you don't need to know the why, you just work around it (by nasal breathing!)
Here's some more that are more about the sleep apnea side of things:
Point being: before you think you (or your kid) is "ADHD", see to it that they are breathing and sleeping properly. They're just so tired they become irritably unstable and unfocused. Lack of sleep = kids are a mess