Objective Methylphenidate is a first-line treatment for ADHD; its contribution to sleep problems in adult ADHD is currently unclear. This study investigates (a) subjective sleep disturbances in a group of initially stimulant medication-naive adults with ADHD and (b) reported changes in sleep problems after 6 weeks of methylphenidate treatment. Method A prospective, non-randomized, non-blinded, 6-week follow-up study utilising a self-report measure. Results We found (1) a large difference in reported sleep quality between methylphenidate medication-naive patients and controls at baseline, (2) a marked improvement in patients after methylphenidate medication, and (3) largest improvement for patients with the poorest reported sleep at baseline. Conclusion The study indicates that treatment with methylphenidate increases subjective sleep quality for at least some adults with ADHD.