The Collaborative International Dictionary of English v.0.48:

Slumberous \Slum"ber*ous\, a.
   1. Inviting slumber; soporiferous. "Pensive in the slumberous
      shade." --Pope.
      [1913 Webster]

   2. Being in the repose of slumber; sleepy; drowsy.
      [1913 Webster]

            His quiet and almost slumberous countenance.
                                                  --Hawthorne.
      [1913 Webster]

WordNet (r) 3.0 (2006):

slumberous
    adj 1: quiet and tranquil; "a slumberous June morning" [syn:
           slumberous, slumbrous]
    2: 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:

18 Moby Thesaurus words for "slumberous":
   comatose, dopey, dozy, drowsy, heavy, hebetudinous, hypnotic,
   narcotic, nodding, opiate, sluggish, snoozy, somnifacient,
   somniferous, somnolent, soporific, stupid, torpid