Summary In the present study, we assessed the effects of regular use of methylphenidate medication in children diagnosed with attention deficit hyperactivity disorder (ADHD) on sleep timing, duration and sleep architecture. Twenty-seven children aged 6–12 years meeting diagnostic criteria for Diagnostic and Statistical Manual version IV ADHD and 27 control children matched for age (±3 months) and gender. Two nights of standard polysomnographic (PSG) recordings were conducted. ADHD children were allocated randomly to an on- or 48 h off-methylphenidate protocol for first or second recordings. Control children’s recordings were matched for night, but no medication was used. Mixed modelling was employed in the analyses so that the full data set was used to determine the degree of medication effects. Methylphenidate in ADHD children prolonged sleep onset by an average of 29 min [confidence interval (CI) 11.6, 46.7], reduced sleep efficiency by 6.5% (CI 2.6, 10.3) and shortened sleep by 1.2 h (CI 0.65, 1.9). Arousal indices were preserved. Relative amounts of stages 1, 2 and slow wave sleep were unchanged by medication. Rapid eye movement sleep was reduced (−2.4%) on the medication night, an effect that became non-significant when control data were incorporated in the analyses. PSG data from ADHD children off-medication were similar to control data. Our findings suggest that methylphenidate reduces sleep quantity but does not alter sleep architecture in children diagnosed with ADHD. An adequate amount of sleep is integral to good daytime functioning, thus the sleep side effects of methylphenidate may affect adversely the daytime symptoms the drug is targeted to control.