The purpose of this narrative review was to synthesize the existing literature on the impact of sleep on daytime functioning in both typically developing (TD) children and children with attention-deficit/hyperactivity disorder (ADHD). Correlational studies in children suggest that insufficient sleep and impaired daytime functioning are significantly associated; however, this does not address the causal relationships between sleep and daytime functioning. The review results indicated that there is limited experimental sleep manipulation research in children. In the eight studies that employed experimental methods to examine sleep restriction, the consequences of insufficient sleep were greatest for attention and inconsistent for other domains, such as cognition and emotion regulation. Despite the significant co-occurrence of ADHD and sleep problems, the experimental sleep research focused on the daytime impact of shorter sleep in children with ADHD is extremely limited and as such more research is needed.