Cognitive function in central disorders of hypersomnolence.

Central disorders of hypersomnolence (CDH), a group that includes narcolepsy type 1 (NT1), narcolepsy type 2 (NT2), idiopathic hypersomnia (IH), and Kleine-Levin syndrome (KLS), are defined by excessive daytime sleepiness (EDS) and prolonged or fragmented sleep, but their impact extends well beyond wakefulness. Cognitive dysfunction is a core, disabling feature of the CDH spectrum, affecting attention, processing speed, memory, and executive function to varying degrees across subtypes. This chapter reviews current evidence on the nature, magnitude, neurobiological mechanisms, comorbidity interactions, treatment responsiveness, and clinical consequences of cognitive impairment in CDH, and aims to give the clinical neuroscientist and sleep specialist a practical framework.