A neurological disorder, it affects people by making them excessively sleepy at unpredictable times. People who have it may find it unbearably hard to stay awake during meetings, music recitals, while driving, during a conversation, whenever. Then, at night, it may be difficult to get restful sleep, in part because of vivid dreams. You didn't mention the other condition usually associated with narcolepsy, called cataplexy, a sudden loss of voluntary muscle control without warning, usually triggered by strong emotion. People with cataplexy suddenly fall to the ground when anything makes them laugh or cry.
Physical exercise can cause the reaction too, which usually lasts for just a few seconds.
About 1 in 2,000 people have narcolepsy. Lots of times it's not diagnosed because people ignore their chronic sleepiness. There are also mild forms of cataplexy that cause people to drop objects or sit down suddenly. The root cause of narcolepsy is quite a mystery to neurologists. At Stanford University, researchers are studying a colony of Doberman pinschers, Labrador retrievers, and mixed breeds that have inherited the condition.