Introduction: The primary aim of this retrospective study was to assess differential associations between sleep disturbances with externalizing/internalizing problems, ADHD profiles (inattention, hyperactivity/impulsivity), and social impairment among children with ADHD, autism spectrum disorder (ASD), and comorbid diagnoses (ASD+ADHD). Methods: Caregivers of patients with ADHD (N = 28), ASD (N = 29), and ASD+ADHD (N = 57) completed the Child Behavior Checklist, Conners 3rd Edition, and Pediatric Sleep Questionnaire to assess externalizing/internalizing problems, social functioning, ADHD presentations, and sleep disturbances. Results: Findings indicated that among the ASD+ADHD group, select sleep factors contributed to 17% of the variance in externalizing problems, 16% in hyperactivity/impulsivity, and 15% in inattention problems, despite controlling for intellectual functioning and stimulant use; whereas no associations were observed among ASD or ADHD groups. Conclusions: In brief, sleep may impact behavioral and attention dysregulation more intimately among those with ASD+ADHD, underscoring the need to incorporate sleep screening measures in diagnostic evaluations and in treatment considerations for behavioral dysregulation.