Narcolepsy is characterised by chronic excessive daytime sleepiness, often with sudden loss of muscle tone. Other symptoms include sleep paralysis and hallucinations. The cause of the disorder is unknown, but narcolepsy may be an autoimmune disease.
Children of patients with narcolepsy have a 40-fold increased risk, suggesting a genetic cause. Narcolepsy is strongly associated with specific HLA haplotypes. However, studies show low concordance in twins (25%). Environmental factors may have a prominent role, triggering the disease. As the neuropeptide hypocretin-1 is deficient in most human patients, the cause may be a HLA-associated autoimmune destruction of hypocretin-containing neurons in the lateral hypothalamus.