ADHD is one of the most common childhood disorders and is associated with negative outcomes and lifelong impairments. Sleep problems are common in children with ADHD and may contribute to, cause or mimic ADHD symptomatology. There is evidence of a relationship between sleep and functional outcomes however, a systematic review conducted as part of this thesis highlighted a lack of longitudinal studies and multi-method assessments of sleep and recommended that future work should include both objective and subjective measures of sleep and that both child and parent reports of sleep should be considered. To address this gap in the knowledge, a longitudinal case-control study was designed to investigate the relationship between sleep and cognitive function, daytime functioning, academic attainment, and psychosocial well-being in children with ADHD and age and sex matched controls. Fifteen children with ADHD and fifteen matched controls were recruited to undergo a comprehensive assessment at two time points 12 months apart. The assessment comprised of measures of sleep, including a polysomnography recording and eight days and nights of actigraphy recording, standardised cognitive and academic assessments, and subjective measures of well-being, quality of life and strengths and difficulties. Children with ADHD showed impairments in a range of domains: sleep, attention, working memory, problem-solving ability, academic performance, physical activity, emotional and behavioural functioning, quality of life, and well-being. Sleep duration was associated with cognitive function, maths ability and psychosocial well-being measures whereas sleep efficiency was associated with physical activity. Children who had poorer quality of sleep, reduced sleep duration and more fragmented sleep made less progress academically. The findings of this thesis raise questions which can help to design future research. It is proposed that physical activity may be an area to target for intervention with the aim of improving sleep and therefore academic performance within this population.