Orexin, also called hypocretin, is a neuropeptide that regulates arousal, wakefulness, and appetite. The most common form of narcolepsy, in which the sufferer briefly loses muscle tone (cataplexy), is caused by a lack of orexin in the brain due to destruction of the cells that produce it. There are approximately 70,000 orexin producing neurons in the human brain that project from the lateral hypothalamus to neurons and brain regions that modulate wakefulness. However, the axons from these neurons extend throughout the entire brain and spinal cord, where there are also receptors for orexin.