Source : Webster's Revised Unabridged Dictionary (1913)

Somnolence \Som"no*lence\, Somnolency \Som"no*len*cy\, n. [L.
   somnolentia: cf. F. somnolence.]
   Sleepiness; drowsiness; inclination to sleep.

Source : WordNet®

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