Narcolepsy

A sleep disorder in which the subject passes immediately into REM sleep, often during the daytime.  A narcoleptic episode is sometimes described as a "sleep attack."  Narcolepsy is characterized by extreme daytime drowsiness, cataplexy, and muscle weakness or paralysis.  An attack is often precipitated by a strong emotion, even laughter.  There is no cure for the disease at present.  Typical treatments involve stimulants and anti-depressants. 

Source:

Dement, The Promise of Sleep