Narcolepsy without cataplexy is characterized by excessive day-time sleepiness associated with uncontrollable sleep urges and sometimes paralysis at sleep, hypnagogic hallucinations and automatic behavior. No epidemiological data are available. Prevalence numbers are controversial, sometimes higher and sometimes lower than those of narcolepsy with cataplexy (see this term), depending on reports. Narcolepsy without cataplexy manifests generally between the age of 10 and 30 years old and is a lifelong disease. The etiology is unknown. Reduction of hypocretin-1 levels is found in the cerebrospinal fluid in 10-20% of cases, and presence of the HLA DQB1*0602 allele is noted in 40% of cases (more than in the general population but less than in cases of narcolepsy with cataplexy). Diagnosis is based on nocturnal and diurnal polysomnography which reveals a mean sleep latency of less than 8 min with at least two episodes of paradoxical sleep. Clinical diagnosis is often difficult due to the absence of major symptoms such as cataplexy. Other causes of sleepiness, including chronic insufficient sleep and idiopathic hypersomnia (especially the form without long sleep time) (see this term), must be systematically taken into account. Treatment is based on stimulant drugs. Modafinil is the first line treatment because it presents the best benefit/risk ratio. This treatment has replaced second-line treatments with methylphenidate or amphetamines. The disease has a negative impact on scholarly and professional performances. Spontaneous evolution of the disease has not been studied.