Background: Sleep disturbance is documented as part of the behavioural phenotype ofAngelman syndrome (AS) and its 'sister' syndrome, Prader-Willi syndrome (PWS). Method: The sleeping patterns of22 children (AS 12, PWS 10), aged 5-13 years, and their parents were investigated using parental reports and actigraphy. Findings: Both groups of children have less sleep than would be expected for children oftheir age, and both groups experience night and early morning wakings. Both groups of children may take some time to fall asleep but children with AS appear to have more severe settling difficulties and require parental intervention, unlike children with PWS. Data suggest that parents' sleep is disturbed by children's sleeping patterns in both groups, although parents ofchildren with PWS may be less aware of the severity oftheir child's sleep disturbance. Conclusions: There are a number offactors which limit the conclusions of this study, primarily the small number ofparticipants. In terms ofclinical practice the findings ofthis study suggest that children diagnosed with the two syndromes should be screened for sleep disturbance. Treatment may include medical intervention for causes such as sleep related breathing disorders or epilepsy, and/or psychological intervention for behavioural factors.