CLOCK and attention deficit-hyperactivity disorder: The prevalence of circadian alterations (affecting 73-80% of ADHD patients), consistency of biological markers across studies (phase delays, altered melatonin and cortisol rhythms, disrupted clock gene expression), and efficacy of circadian-targeted interventions in improving both sleep and core ADHD symptoms support a model wherein circadian disruption may play an important role in ADHD pathophysiology in a substantial subgroup, though evidence on remission of ADHD with circadian interventions is lacking.