Narcolepsy is an incurable non-progressive disease of the central nervous system. In humans, narcolepsy causes excessive drowsiness during the day (sometimes a sleep-attack occurs), cataplexy (sudden loss of muscle tone), hallucinations, and sleep paralysis. In the horse and other mammals cataplexy is the most frequently observed symptom. Excessive drowsiness can occur but is harder to observe. Cataplexy is caused by a fragmentation of the REM sleep. The etiology of narcolepsy is still subject to debate, partly because normal sleeping patterns are poorly understood. In humans and certain breeds of dogs a hereditary background has been demonstrated. In Shetland ponies the disease runs in certain families. The role of trauma and infection is the subject of debate. Cataplexy (which can be induced by physostigmine injection) confirms the diagnosis. Several drugs are available for the treatment of narcolepsy in humans. However there are a few data on the results of treatment of narcolepsy in the horse.