Objective: Attention-deficit/hyperactivity disorder (ADHD) is a common neurodevelopmental disorder with early onset. Sleep disturbances in children with ADHD may impair attention and learning and exacerbate symptoms. Maintaining good sleep is crucial in ADHD treatment. This study investigated the effects of ADHD medications on subjective and objective sleep in children under 18, employing actigraphy and a questionnaire. Methods: This single-centre prospective observational study enrolled patients under 18 receiving methylphenidate (MPH), lisdexamfetamine (LDX), atomoxetine (ATX), or guanfacine (GXR) between June 1 and August 31, 2022. Patients using other drugs were excluded. Subjective sleep was assessed using the Athens Insomnia Scale (AIS), and objective sleep variables were analysed using an actigraph worn on the waist. Results: A total of 21 patients were enrolled. AIS total scores of 4 or more were significantly more common in GXR-treated patients (P = 0.009) than in ATX-treated patients. However, other variables were not statistically significant. Among patients receiving non-psychostimulant monotherapy, sleep time was longer (P = 0.045), and postural changes were more frequent (P < 0.001) than in those receiving combined psychostimulant and non-psychostimulant therapy. Conclusions: Findings suggest ATX may improve subjective sleep more effectively than GXR. A trend toward shorter sleep period time was observed in the combined group, likely due to the arousal effects of psychostimulants. Reduced postural changes in the combined group may indicate altered sleep structure.