Children with neurodevelopmental disorders commonly experience sleep problems. Williams Syndrome (WS), a rare genetic disorder characterised by a complex, uneven cognitive profile, is no exception. Compared with children with typical development (TD), school-aged children with WS experience significant sleep disruption: shorter sleep duration, more night wakings, greater bedtime resistance and excessive daytime tiredness. In children with TD, sleep problems impede optimal daytime functioning. In WS, this could compound existing difficulties. Few studies have examined sleep in very young children with WS and little is known about the early emergence of sleep problems in this population. To date, studies have been based on parent-report and no studies have objectively assessed sleep patterns using longitudinal approach in toddlers with WS. Thus, the current study sought to objectively explore sleep patterns in toddlers with WS. Parents of 38 children (13 WS, 25 TD) completed the Brief Infant Screening Questionnaire and the Medical and Demographics Questionnaire and sleep patterns were assessed using actigraphy. Data were collected longitudinally at ages 18, 24 and 30 months. Significant sleep disturbances were present in WS from 18 months old. Sleep duration, as measured by actigraphy, was significantly shorter in WS at all ages and, furthermore, parents of children with WS reported more night wakings, longer settling times and high levels of parental involvement. Crucially, whereas actigraphy showed developmental improvements in sleep quality in TD, no longitudinal changes were found in WS. Findings could be instrumental in working towards instigating appropriate, timely sleep management in this group, thus improving outcomes for children and their families.