Hypersomnolence is a general term for diseases in which excessive sleepiness occurs during the time when people are originally awake and active, and they repeatedly fall asleep. It includes central hypersomnia and sleep apnea syndrome, which is considered to be secondary hypersomnia due to repeated sleep interruptions. In addition to excessive daytime sleepiness, prolongation of nighttime sleep of 9 hours or longer is defined as hypersomnolence disorder in the DSM-5. Central hypersomnia includes narcolepsy, idiopathic hypersomnia, and recurrent hypersomnia. The mechanism of idiopathic narcolepsy is due to the loss of the orexin nervous system, but there are also symptomatic pathologies, categorized as symptomatic narcolepsy, which are found in those with hereditary diseases, brain tumors, demyelinating diseases, degenerative diseases, head trauma, and paraneoplastic brain syndrome. Most cases are associated with hypothalamic lesions, with neuromyelitis optica being the most common. Symptomatic narcolepsy can be diagnosed by the presence of hypersomnia and decreased orexin levels based on the International Classification of Sleep Disorders, 2nd Edition. Among the types of symptomatic narcolepsy, the mechanism involving anti-Ma-2 and AQP4 antibodies in the immune system that causes neuromyelitis optica was recently clarified. On the other hand, the hypersomnia symptoms observed in thalamic infarction are not considered to be related to orexin systems. Although there is no curative therapy for idiopathic narcolepsy, in the case of symptomatic narcolepsy, hypersomnia can be improved if the underlying disease can be treated (steroid pulse therapy, etc.). Receptors are also conserved in idiopathic narcolepsy; therefore, orexin neuronal regeneration and agonist therapy may be new therapeutic options. Orexin remains an important marker for optimal treatment selection.
 Authors' abstract