The Collaborative International Dictionary of English v.0.48:

Somnolent \Som"no*lent\, a. [F. somnolent, L. somnolentus, from
   somnus sleep, akin to Gr. ?, Skr. svapna sleep, dream, svap
   to sleep, Icel. sofa, AS. swefn sleep. Cf. Hypnotic,
   Somnambulism, Soporific.]
   Sleepy; drowsy; inclined to sleep. -- Som"no*lent*ly, adv.
   [1913 Webster]

         He had no eye for such phenomena, because he had a
         somnolent want of interest in them.      --De Quincey.
   [1913 Webster]

WordNet (r) 3.0 (2006):

somnolent
    adj 1: inclined to or marked by drowsiness; "slumberous (or
           slumbrous) eyes"; "`slumbery' is archaic"; "the sound had
           a somnolent effect" [syn: slumberous, slumbery,
           slumbrous, somnolent]

Moby Thesaurus II by Grady Ward, 1.0:

85 Moby Thesaurus words for "somnolent":
   anesthetized, apathetic, benumbed, blase, bored, cataleptic,
   comatose, dead, debilitated, doped, dopey, dormant, dozy, dreamy,
   droopy, drowsy, drugged, drugged with sleep, dull, enervated,
   exanimate, half asleep, heavy, heavy with sleep, heavy-eyed,
   hebetudinous, in a stupor, inactive, inanimate, inert, jaded,
   lackadaisical, languid, languorous, leaden, lethargic, lifeless,
   listless, lumpish, moribund, napping, narcoleptic, narcose,
   narcotic, narcotized, narcous, nodding, numb, opiate, oscitant,
   out of it, passive, phlegmatic, pooped, sated, sedated,
   sleep-drowned, sleep-drunk, sleep-filled, sleep-swollen, sleepful,
   sleepy, slow, sluggish, slumberous, slumbery, snoozy, somnifacient,
   somniferous, soporific, stagnant, stagnating, stretchy, stultified,
   stuporose, stuporous, supine, torpid, vegetable, vegetative, wan,
   weary, world-weary, yawning, yawny