Someone who's narcoleptic has a disorder that makes them fall asleep very abruptly in the middle of the day. A narcoleptic bus driver could be dangerous.
Narcoleptic can be an adjective or a noun, describing the condition of narcolepsy or a person who suffers from it. A narcoleptic has a sleep disorder that makes him exhausted, even after a full night's sleep. Some narcoleptics drop off to sleep in the middle of a sentence, while others just become very sleepy throughout the day. A French doctor coined the word narcolepsie from the Greek roots narke, "numbness or stupor," and lepsis, "attack or seizure."