Sleep problems are commonly reported in attention deficit hyperactivity disorder (ADHD), and are also a familiar characteristic of typical development (TD). We sought to elucidate the relationship between sleep, ADHD trait behaviors, and cognitive inattention, and how it manifests between ADHD and TD children. Participants: Eighteen children diagnosed with ADHD and 20 age-matched TD controls aged 5 to 11 years old participated in the study. Methods: Sleep profiles were assessed using Children’s Sleep Habits Questionnaire and actigraphy measures. Behavioral functioning was examined using Conners’ Parent Report Scale and attention using the computerized Conners’ Continuous Performance Task. Results: We found evidence of (a) poorer sleep quality in the ADHD group, despite no difference in actual sleep time, (b) poor sleep quality in TD children predicting increased ADHD-trait behaviors, despite no association with attention, and (c) a consistent trend for poor sleep quality predicting reduced attentional control in ADHD children, despite no association with behavior. Conclusions: Poor sleep quality affects developmental subgroups in different ways. For ADHD children, poor sleep worsens their predisposed attentional deficit, while for TD children it mimics ADHD behaviors. These findings have important implications for the debate on overdiagnosis of childhood ADHD, and the use of sleep-based interventions. Above all, they highlight the importance of promoting good sleep hygiene in all children.