Narcolepsy is the second leading cause of excessive daytime sleepiness. Narcolepsy occurs in approximately 1 person per thousand. In many cases, however, diagnosis is not made until many years after the onset of symptoms due to the fact that patients assume that sleepiness is not indicative of a disease. Narcolepsy is a very disabling and underdiagnosed illness. The effect of narcolepsy on its victims is devastating. Studies have shown that even after treatment narcoleptic patients are often markedly psychosocially impaired. A large majority of narcoleptic patients are still undiagnosed, and most narcoleptic people are diagnosed after 14 years of syptoms. Since the symptoms of narcolepsy usually appear during adolescence, most narcoleptic people are diagnosed too late to prevent the dramatic impact of the disease. The main symptoms of narcolepsy are excessive daytime sleepiness and abnormal REM sleep. Narcolepsy is a serious and common medical problem. Other disabling symptoms of narcolepsy are sleep paralysis, cataplexy and hypnagogic hallucinations, are pathological equivalents of REM sleep. Narcolepsy can be diagnosed using specific medical procedures. The diagnosis of narcolepsy is usually easy if all the symptoms of the illness are present. However, the symptoms of dissociated REM sleep such as cataplexy are mild. Diagnosis usually comes from a nocturnal polysomnogram and a multiple sleep latency test (MSLT).