This article investigates the impact of sleep problems on preschool-aged children - in particular the relationships between sleep problems and child behaviour, health-related quality of life, verbal, pre-literacy, and early numeracy skills, diagnosis of attention-deficit/hyperactivity disorder, and injury. Data was analysed from the Longitudinal Study of Australian Children, a nationally representative study of 4983 Australian children aged 4 to 5 years. The analysis found that, compared with children without sleep problems, children with sleep problems had poorer child health-related quality of life, more behaviour problems, and higher rates of attention deficit hyperactivity disorder. Difficulty going to sleep and morning tiredness had greater adverse associations than snoring or night waking. These findings have implications for school readiness.