This thesis, which focuses on sleep disturbance in people with neurodevelopmental disabilities, is divided into three sections. Paper one is a systematic review of the extant literature on objective studies of sleep in neurodevelopmental genetic disorders. Twenty papers met inclusion criteria and were subject to quality assessment, of which five were found to be high-quality, thirteen were medium-quality and two were low-quality. Studies were grouped by disorder and although there was some disparity across investigations, generally there was agreement about specific sleep difficulties in each disorder which seem to be part of the behavioural phenotypes. Overall a lack of total sleep, diminished REM sleep, and fragmented, less efficient sleep are prevalent across the disorders. Paper two is an empirical study which employed actigraphy to assess sleep in children with mucopolysaccharidosis type III (MPS III) and typically developing children. Parents completed a sleep diary, a sleep questionnaire and took saliva samples from their child. Actigraphic findings showed that MPS III patients had lengthened sleep onset latencies and greater daytime sleep than controls, but night-time sleep duration was within the normal range. In the MPS III group, some sleep problems correlated with age and progression of the disorder. Analysis of saliva samples revealed that children with MPS III had abnormal melatonin concentrations. Questionnaire responses demonstrated that children with MPS III had more sleep difficulties in all domains compared to controls. Implications for the management of sleep difficulties are discussed. Paper three is a critical appraisal of the research process which includes personal reflections on designing and conducting this research and a discussion of the principal issues which arose. Strengths and limitations of the research, ideas for further research and implications for clinical practice are considered.