What Causes Narcolepsy? Narcolepsy with cataplexy is caused by a lack of the chemical hypocretin in the brain. Hypocretin is an important chemical for regulating wakefulness and rapid eye movement (REM) sleep. Narcolepsy without cataplexy includes most of the same symptoms, but its cause is unknown. Injuries to the hypothalamus and brain stem, tumors, and stroke are under investigation.