APA Dictionary of Psychology

n. excessive sleepiness during daytime hours or abnormally prolonged episodes of nighttime sleep. This can be a feature of certain dyssomnias (e.g., narcolepsy) or other sleep or mental disorders, or it can be associated with neurological dysfunction or damage, a general medical condition, or substance use. However, hypersomnia may occur in the absence of any known cause or association with another condition (see primary hypersomnia). See also disorders of excessive somnolence. Compare hyposomnia.