The vertebrobasilar arteries are the blood vessels in the back of the neck, next to the spinal cord, that supply the brainstem -- the portion of the brain that, among other things, is responsible for maintaining consciousness. A blockage of blood flow in the vertebrobasilar arteries can cause syncope.
1. NREM-a recurring sleep state during which rapid eye movements do not occur and dreaming does not occur; accounts for about 75% of normal sleep time. nonrapid eye movement, nonrapid eye movement sleep, NREM sleep, orthodox sleep. sleep, slumber-a natural and periodic state of rest during which consciousness of the world is suspended; he didn't get enough sleep last night; calm as a child in dreamless slumber. 1  Facebook.
REM sleep is an active mind inside a paralyzed body, while nREM sleep is a resting mind inside a functioning body. This is why sleepwalking is possible during nREM sleep and not during REM sleep: your body is paralyzed during REM sleep but not in nREM. As for why sleepwalking occurs... well, we still don't really know. See Wikipedia: Sleepwalking for much of the current knowledge.