Studies have connected sleep disorders to various mood issues and ADHD. In fact, according to a 2009 article in the Journal of the Canadian Academy of Child and Adolescent Psychiatry:
Clinical experience, as well as empirical evidence from numerous studies and case reports, have demonstrated that childhood sleep disorders that are primarily medical in etiology, such as obstructive sleep apnea (OSA), restless legs syndrome (RLS), and Delayed Sleep Phase Disorder, and those that are related to behavioral or environmental factors, such as behavioral insomnia of childhood and voluntary sleep curtailment, may present primarily with daytime sleepiness and neurobehavioral symptoms, many of which overlap with the cardinal symptoms of ADHD. These observational and treatment studies have demonstrated that insufficient and poor quality sleep related to primary sleep disorders result in daytime sleepiness and behavioral/mood dysregulation, and impact on neurocognitive functions in children. The two primary sleep disorders about which there exists the most empirical evidence (OSA and RLS/periodic limb movement disorder PLMD) regarding impact on daytime functioning are discussed below.
This makes sense since different parts of the brain are activated and suppressed during the four phases of the sleep cycle. This ebb and flow in brain activity is believed to be one of the mechanisms involved in attention, learning, and memory. Various neurotransmitters predominate these phases to allow for this integration during sleep. Some of these neurotransmitters, or brain signals, are linked to the ability to focus and learn.
Furthermore, lack of sleep can cause mood imbalances such as depression, fatigue, and anxiety. This can create issues with attention, focus, and learning, and be linked to ADHD.