Sleep deprivation studies have highlighted the importance of adequate sleep for optimal daytime functioning. However, there is limited research exploring whether variations in natural sleep produce similar difficulties. This thesis explored the relationship between naturalistic sleep and cognitive control in children, adolescents, and young adults. Developmental differences in sleep and cognitive control were observed and findings highlighted the importance of obtaining adequate sleep, as daytime sleepiness and sleep duration were associated with poorer performance and inefficiencies in error monitoring. A high proportion of young people were also found to experience inadequate sleep, further emphasising the need to address this widespread issue.