Answer :
(Okay, why is this question in the World Lang section??) The sudden and involuntary sleeping condition is called narcolepsy. It's a psychological problem and is often mentioned along with sleep apnea, sleep terrors and nightmares. Narcolepsy doesn't follow the normal chart for falling into REM sleep - through stage 1,2,3,4 and all - but rather it falls directly into REM.
