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Adjective: slumberous  slúm-b(u-)rus
  1. Inclined to or marked by drowsiness
    "slumberous eyes";
    - slumbery [archaic], slumbrous, somnolent
     
  2. Quiet and tranquil
    "a slumberous June morning";
    - slumbrous

See also: asleep, relaxing, reposeful, restful, sleeping