Narcolepsy is a sleep disorder. It happens when the nervous system does not work properly. People with narcolepsy tend to fall asleep during the daytime, even if they do not want to. Very often, they do not sleep well at night, and during the day they fall asleep uncontrollably. There are two different forms. The classical (more common) form is the one with muscle failure (called cataplexy). There is also a less common form without cataplexy.