Anectodal. Feel free to shoot it down.
Walking is an acquired skill; As far as I'm concerned exhaustion causes disorientation. Even after waking up it sometimes takes a second or so to get balanced.
How is it sleepwalkers don't stumble around when walking?
Balance is mediated by 3 things: the cerebellum, vision and the ear via the semicircular canal + vestibular nerve (i.e. the ear).
Sleepwalkers lack the visual feedback, but they have a functional cerebellum and semicircular system which is sufficient.
As for waking up and feeling disorientated, if that's when you get up fast that's because your semicircular system hasn't yet reset (as well as your blood pressure has just dipped).
Mind all of you, even the cerebellum sleeps. The waves that are generated in the neocortex predict the same waves in the cerebellum. There probably is no active vision, but the activation patterns of the dreaming visual cortex may give the sleep walking person an idea of the world around him/her. This might, however imply that a great deal of luck is involved in getting somewhere in one piece (given that the patient even moves outside...). During normal REM sleep we speak of complete muscle atonia, the eyes not included. However, this does not work that well in all patients, meaning that they start living their dream. Mind you as well that keeping balance is partially programmed in the muscular innervation (connection patterns in the spinal cord that registers and partially controls muscle tone of agonists and antagonists), and in part by the cerebellum indeed. The cerebellum, however can be devided in a cortex and in cerebellar nuclei. The cortex is not involved in keeping balance as such, much of this is programmed by the cerebellar and pontine nuclei.
The part of the brain that maintains balance acts subconsciously, and so while you are sleeping in the behavioral sense, you part of your brain (the cerebellum) do not necessarily have to be asleep. And so it can still maintain balance.
