Causes. It’s not known exactly what causes narcolepsy, but it’s thought to be an inherited autoimmune disease involving a deficiency of hypocretin, a chemical that helps the mind stay awake. This neurotransmitter is made in the region of the brain called the hypothalamus. Hypocretin acts on neurons to control sleepfulness and wakefulness. In Type 1 narcolepsy, hypocretin levels are low and cataplexy appears along with the disorder. Type 2 narcolepsy is not characterized by abnormally low levels of hypocretin and involves excessive daytime sleepiness without cataplexy. A third type of narcolepsy can be caused by a brain tumor or brain injury.