I don't know about studies. But I have a PG child with ADD that has never slept well, but then developed anxiety and insomnia. The anxiety was unmedicated, but we did therapy.
As the anxiousness and the insomnia worsened, the child became grouchy, and at that point we saw a psychiatrist.
We were told that depression presents itself in young children as grouchiness. An anxious person can become depressed when anxiety is untreated. The doctor's response was to improve the mood by treating the anxiety, and then address the ADD.
So many of these problems overlap; anxiety does make ADD worse. And sometimes anxiety is misdiagnosed as ADD/ADHD, and sometimes people have both diagnoses.
