Objective: To evaluate children with ADHD and sleep problems with polysomnography (PSG) after guanfacine extended-release (GXR) administration. Method: Double-blind, randomized, placebo-controlled study was terminated early due to treatment-emergent concerns after enrolling 29 children aged 6 to 12 years. After >4 weeks dose adjustment and >1 week dose stabilization, 11 children received GXR and 16 controls underwent analyses with PSG. Results: Although GXR improved ADHD symptoms, the primary outcome variable, total sleep time, was shorter in contrast to placebo (–57.32, SD = 89.17 vs. +31.32, SD = 59.54 min, p = .005). Increased time awake after sleep onset per hour of sleep was the primary factor for the reduction. Although rapid eye movement (REM), non-REM, and N3/slow wave sleep times were reduced, these were proportional to the overall sleep reduction. Sedation was common with GXR (73% vs. 6%). Conclusion: Morning-administered GXR resulted in decreased sleep and may contribute to sedation.