The kind of involuntary
sleep you experience when you'
re really tired and trying to stay awake in a monotonous situation, like a class lecture. One minute you're propping your
head in your hand, trying to pay attention, when all of a sudden you're out -- at least until your elbow slips sideways or your
head falls forward, jarring you back to attention. Caused by a consistent lack of sleep over several days. Often the cause of car accidents, when the driver nods off during long hauls.