This is because narcolepsy is a disorder of REM sleep. REM sleep is the stage when dreaming is most likely to occur. When most people drift off they do not usually reach their first period of REM sleep until about 90 minutes after they have fallen asleep. Narcoleptics however will often go directly into REM sleep. The recurrent sleepiness eventually becomes debilitating.
Many narcoleptics still remain undiagnosed.
The disorder tends to develop in the early teenage years. There is often a hereditary component present as well. Genes which play a role in the disorder have been identified. There is no relationship between narcolepsy and gender. Although there are some stimulant medications which are helpful, the search for a cure continues.