The Collaborative International Dictionary of English v.0.48:

Somnambulist \Som*nam"bu*list\, n.
   A person who is subject to somnambulism; one who walks in his
   sleep; a sleepwalker; a noctambulist.

   Syn: somnambulator.
        [1913 Webster]

WordNet (r) 3.0 (2006):

somnambulist
    n 1: someone who walks about in their sleep [syn: sleepwalker,
         somnambulist, noctambulist]