Sleepwalking.

There is a natural occuring chemical in the head that prevents us from acting out our dreams, or moving about, possibly inflicting damage upon ourselfs unconsciously. The chemical basically paralyses us during REM.
Sleepwalking may be described as an imbalance, partial abscence or disorder of this chemical (i dont know what its called ).

Im also not sure what period of sleep we 'sleepwalk' in...who knows that?