The aim of this study is to understand adolescent brain maturation by analysing developmental trajectories of delta and theta EEG activity during sleep. The study followed participants in three cohorts: C6, C9, and C12. Participants were recruited using newspaper advertisements and word of mouth in the United States of America and were aged 6 to 16 years at baseline assessment. Participants completed assessments every six months. The C6 cohort was followed up for up to four years, C9 cohort for seven years and C12 for six years.