It is a sleep disorder characterized by four classic symptoms: excessive daytime sleepiness, cataplexy, sleep paralysis, and hypnagogic hallucinations. The sleepiness can be profound and may lead to falling asleep in inappropriate situations. Cataplexy is a loss of muscle tone in response to an emotional stimulus, such as surprise or laughter. Sleep paralysis is an inability to move one's body while being awake, usually while falling asleep or awakening. Hypnagogic hallucinations are vivid dream-like auditory, visual, or tactile sensations that occur while falling asleep.