The Collaborative International Dictionary of English v.0.48:

Somnolence \Som"no*lence\, Somnolency \Som"no*len*cy\, n. [L.
   somnolentia: cf. F. somnolence.]
   Sleepiness; drowsiness; inclination to sleep.
   [1913 Webster]

WordNet (r) 3.0 (2006):

somnolence
    n 1: a very sleepy state; "sleepiness causes many driving
         accidents" [syn: sleepiness, drowsiness, somnolence]
         [ant: sleeplessness, wakefulness]