Background The sleep problems of children with intellectual disabilities remains a relatively neglected topic in spite of the consistent reports that such problems are common, often severe and persistent with potentially serious consequences for the children and their families. Children with Down syndrome (DS) are a case in point. They often suffer from obstructive sleep apnoea (OSA), early detection of which is recommended because of its potentially adverse effects on development. This study is concerned with aspects of assessment that have been considered important in helping to recognise OSA in children with DS. The relationships between different objective measures, and between these measures and parental reports of their child's sleep and daytime behaviour, were explored. Method Overnight recordings were carried out on a group of children with DS (n = 31) involving video and audio recording, oximetry and activity monitoring during sleep. Parents also completed questionnaires concerning their child's sleep and daytime behaviour. Results Parents' reports of restless sleep and noisy breathing were supported by objective measures of activity during sleep and audio recording respectively. No significant association was found between objective measures of restlessness during sleep and ‘snoring’ (see later for definition), nor were objective measures of restlessness related to reductions in overnight blood oxygen levels. However, the objective measure of snoring was significantly associated with reductions in overnight blood oxygen levels. All three of the objective measures were significantly associated with parental reports of various types of disturbed daytime behaviour. Conclusions The findings have implications for aspects of screening for OSA in children with DS and for the interpretation of the relevance of the results to the children's daytime behaviour.