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]

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "somnolence":
   acedia, apathy, boredom, doziness, drowsiness, dullness,
   enervation, ennui, fatigue, heaviness, hebetude, inanimation,
   indifference, jadedness, lackadaisicalness, languidness,
   languishment, languor, languorousness, lassitude, lenitude, lentor,
   lethargy, lifelessness, listlessness, oscitancy, oscitation,
   pandiculation, passivity, phlegm, satedness, sleepiness, sloth,
   slothfulness, slowness, sluggishness, somnolency, stretching,
   stupefaction, stupor, supineness, torpidity, torpidness, torpitude,
   torpor, weariness, world-weariness, yawning