The objective of this study was to investigate potential relationships between two measures of sleep impairments (i.e., sleep duration and sleep efficiency [SE]) and attention and executive functioning in children with attention-deficit/hyperactivity disorder (ADHD). Parents of 43 children (mean age = 10 ± 1.8 years) with ADHD completed sleep and behavioral questionnaires. Children also wore a wrist actigraph for seven nights and were subsequently assessed with the Conners' continuous performance test (CPT)-2. A significant relationship was found between lower SE and increased variability in reaction time on the CPT. Shorter sleep duration was associated with a range of executive functioning problems as reported by the parents. The relationships between sleep duration and the executive functioning measures held even after controlling for age, gender, and use of medication, but not the relationships with SE. These results suggest that sleep quantity is an important correlate of executive functioning in children with ADHD.