While attention-deficit hyperactivity disorder (ADHD) can co-occur with hypersomnia and other sleep disorders, it is unknown whether ADHD actually causes hypersomnia. One study conducted in 2006 discovered that children with ADHD were more likely than other children to have daytime sleepiness, which is the main symptom of hypersomnia. Another study in 2004 found that half of children with ADHD had sleep-disordered breathing.