The main purpose of this study is to examine the effects of atomoxetine on ADHD-related insomnia. Atomoxetine (Strattera®) is a non-stimulant drug used to treat ADHD symptoms in both children and adults, and there is evidence that it may also have a positive effect on sleep in children with ADHD. During the study, participants will receive either atomoxetine or placebo for a period of four weeks. We expect that the effects of atomoxetine on sleep will differ from those of placebo, with atomoxetine having a greater effect on improving sleep difficulties.