Source : Webster's Revised Unabridged Dictionary (1913)

Drowsiness \Drow"si*ness\, n.
   State of being drowsy. --Milton.

Source : WordNet®

drowsiness
     n : a very sleepy state; "sleepiness causes many driving
         accidents" [syn: {sleepiness}, {somnolence}] [ant: {wakefulness}]