Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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