The Collaborative International Dictionary of English v.0.48:

Sleepwalker \Sleep"walk`er\, n.
   One who walks in his sleep; a somnambulist.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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