Narcolepsy is a chronic neurological disorder caused by the brain's inability to regulate sleep-wake cycles normally. At various times throughout the day, people with narcolepsy experience irresistable bouts of sleep. If the urge becomes overwhelming, individuals will fall asleep for periods lasting from a few seconds to several minutes. In rare cases, some people may remain asleep for an hour or longer. In addition to excessive daytime sleepiness (EDS), three other major symptoms frequently characterize narcolepsy: cataplexy, or the sudden loss of voluntary muscle tone; vivid hallucinations during sleep onset or upon awakening; and brief episodes of total paralysis at the beginning or end of sleep. Narcolepsy is not definitively diagnosed in most patients until 10 to 15 years after the first symptoms appear. The cause of narcolepsy remains unknown"

Source : National Institute of Neurological Disorders and Stroke

As well as the cause remaining unknown, the prevalence of narcolepsy worldwide is also unclear. With figures ranging from 0.2 to 590 per 100,000 people. (source)

Although at present the disorder has no 'cure', it can be treated. One of the most commonly used treatment drugs is Modafinilplugin-autotooltip__plain plugin-autotooltip_bigModafinil

Modafinil (sold under the brand names Alertec, Alertex, Altasomil, Aspendos, Forcilin, Intensit, Mentix, Modafinil, Modafinilo, Modalert, Modanil, Modasomil, Modvigil, Modiodal, Modiwake, Movigil, Provigil, Resotyl, Stavigile, Vigia, Vigicer, Vigil, Vigimax, Wakelert and Zalux - which itself has an unknown 'mechanism of action'.