Background Sleep impairments frequently co-occur in children with Attention Deficit Hyperactivity Disorder (ADHD), and the nature of their relationship is bidirectional. Sleep problems in this population manifest as difficulties falling asleep, maintaining sleep and in poorer sleep quality, greater daytime sleepiness, altered sleep duration and increased limb movement in sleep. These concerns affect the quality of life, academic performance, cognitive functions, behavioural and family health of the child, negatively impacting their functional outcomes. Early identification and management of sleep problems in this population therefore has deep-rooted clinical utility. In this thesis we aimed to comprehensively delineate the nature of sleep problems in children with ADHD, explore the possible ADHD related cognitive/behavioural facets and environmental factors that might be influencing the child’s sleep and translate our understanding to the design of ADHD-specific sleep assessment tool for clinical utility. Methods and Results Chapter 2 reports the systematic review of studies investigating sleep in children between the age of 5-13 years who are diagnosed with ADHD. 148 empirical studies published between 2009-2019 were reviewed and a narrative synthesis was presented categorising studies into five sections. These included studies exploring the nature of these difficulties (subjective reports, sleep macrostructure and microstructure); studies exploring circadian rhythm patterns in this population, consequences of sleep problems, non-pharmacological interventions affecting sleep and ADHD symptoms, and pharmacological interventions affecting sleep in this population. We found that sleep disturbances may worsen behavioral outcomes; moreover, sleep interventions may improve ADHD symptoms, and pharmacotherapy for ADHD may impact sleep. Gaps in research focussed on the need for using mixed methodologies utilizing objective and subjective reports of sleep, designing well powered studies that define the role of sleep in ADHD clinical picture and facilitate assessment and management of sleep problems. Chapter 3 qualitatively investigated the nature of sleep problems and sleep related behaviours in children with ADHD. 26 parents of children diagnosed with ADHD aged between 6-12 years were interviewed about their child’s sleep. Thematic analysis of the interviews generated three broad themes which revolved around facets of children’s sleep difficulties as perceived by parents, the perceived impacts of these difficulties, and steps taken by parents to improve their child’s sleep. Parents expressed that sleep problems can be a significant disruptor for their children’s functioning and the wider household. Parents reported using need-based individualised behavioural and sleep hygiene approaches to counter their child’s sleep problems. Chapter 4 examined the associations of parent-rated sleep problems and sleep timings of pre-adolescent ADHD children with parental insomnia symptoms, ADHD (screener based) features and dysfunctional attitudes and beliefs about sleep (in 120 parent-child pairs). 82% of children exceeded the threshold for a paediatric sleep disorder, and parental insomnia, ADHD symptoms and dysfunctional beliefs about sleep were associated with childrens’s sleep problem scores, and with the subfactors of sleep anxiety and parasomnias. Sleep was poorer for children whose parents were both insomnia probable and had ADHD consistent features, thereby underlying the significant double impact of both on the child. In Chapter 5, a thirty-five-item parent rated sleep problems questionnaire for children with ADHD was developed. This questionnaire, called Childhood ADHD Sleep Scale (CASS), included 5 domains: Bedtime, Behaviours in Sleep, Sleep Quality, Daytime Functions and Impacts on Family, where the respondent has to choose one out of five options for a sleep problem statement (‘strongly agree’, ‘somewhat agree’, ‘neither agree or disagree’, ‘somewhat disagree’, and ‘strongly disagree’). CASS showed acceptable test-retest reliability and good internal consistency. Exploratory factor analysis of the CASS generated the 4-factor reduced CASS including sleep problems and impacts, executive and sensory regulation, daytime functions, and parasomnias. The reduced CASS demonstrated good test-retest reliability and internal consistency. Both unreduced CASS and reduced CASS were compared with scores from Child Sleep Habits Questionnaire (CSHQ) and Brown Executive Functions and Attention Scales (Brown -EFA). Differences in the trends of associations were discussed, to understand the utility of an ADHD specific sleep assessment questionnaire. In Chapter 6, we used an emotional Stroop test to assess the presence of sleep related attentional bias in 155 young adults and examined whether their Stroop test performance and sleep bias scores would associate with their ADHD screener-based symptom scores. Sleep quality scores, insomnia probability scores and social jetlag and chronotype. ADHD consistency scores, and insomnia probability scores were not found to be associated with sleep attentional bias scores. Sleep attentional bias also did not associate with chronotype or social jetlag, but it was found that habitual use of alarm clocks on workfree days did associate with greater sleep attentional bias, indicating that curtailed sleep due to functional demands on these days might increase attention towards sleep related stimulus. Conclusion This thesis highlighted how sleep functioning manifests in the clinical picture of childhood ADHD. The bidirectional relationship between the two entities were explored through varied methodological approaches to draw associations between the child’s environment, their own neurodevelopmental diversity and the accompanying sleep features that define their ADHD specific sleep functioning. We aimed at creating a framework within which sleep problems in ADHD can be understood and utilized for clinical utility, both in terms of assessment and management of these concerns. We also found that social demands can enhance cognitive processing of sleep related stimulus in older cohorts with or without ADHD features.