Sleep disturbances have been found to increase severity of core autism spectrum disorder (ASD) symptoms such as social and communication difficulties and repetitive behaviours. However, very little is known about the impact of sleep on cognitive functioning in ASD. A systematic literature search identified fifteen original studies meeting inclusion and quality assessment criteria. Inter-rater reliability was strong (kappa=0.73). Multiple cognitive functions were negatively affected by sleep disturbances in ASD. This had a significant impact on daily functioning, as well as impacting families/carers. Possible sleep interventions include behavioural/cognitive training, pharmacological treatment, and sensory interventions. Combining objective and subjective sleep measures would help in early diagnosis, which may contribute towards developing interventions to help those with ASD as well as carers and families.