Go to sleep; also, cease paying attention. For example, As soon as the lights were dimmed he fell asleep, or His lectures are so dull that I fall asleep. The literal usage, which uses the verb fall in the sense of "succumb," dates from about 1300; the figurative is several centuries newer. Also see asleep at the switch.
|change from a waking to a sleeping state; "he always falls asleep during lectures" [ant: arouse]|