Summary Attention-deficit/hyperactivity disorder (ADHD) is a highly prevalent disorder occurring in approximately 3–5% of school-aged children. The core symptoms of ADHD are effectively treated with stimulant medications such as methylphenidate; however, there are also negative side effects, including insomnia. It has been suggested that administration of stimulant medication may alter the timing or regularity of circadian motor activity levels. This study aimed to investigate the impact of stimulant medication on the strength and timing of circadian rhythms in 16 stimulant medication-naïve children with ADHD. Participants were monitored for changes in motor activity during a 3-week blinded placebo-controlled medication trial to examine the impact of immediate-release methylphenidate hydrochloride. Motor activity was measured by actigraphy, and 24-h activity profiles were analysed using cosinor analyses to identify measurable changes in circadian rhythms. The children in this sample demonstrated significant increases in motor activity during the sleep-onset latency period. They also showed a significant reduction in relative circadian amplitude and a phase-delay in the timing of the daily rhythm. Clinicians and parents of children being treated with stimulant medication for ADHD should be aware that stimulant medication may cause disruption of sleep/circadian rhythms. Behavioural strategies to improve sleep may be useful for children experiencing these negative effects from medication.