I was watching something about night terrors on the Discovery channel one morning.

They explained that a part of the brain - it's in the back of the head & can be affected by the position of the neck & head during sleep - that controls orientation & spatial perception. Sometimes, when your body is falling asleep, that brain portion gets confused, so people get sensations of movement, 'swooshing', falling, loop-de-loops, etc. This can also be accompanied by flashing lights, 'fireworks' & other visual effects, as the brain's pulse slows & various areas become slightly deprived of oxygen (people who play the 'Choking Game' & get into auto-asphyxiation experience this on a more lethal level) If you're bothered by the spatial sensations, sometimes changing the pressure on your head & neck by rearranging pillows or trying different sleeping positions can relieve it or lessen it.

Along w/REM's (rapid eye movements) we also get Rapid Ear Movements - the little bones & structures in the ear vibrate & can cause auditory hallucinations - most common is hearing your own name being called, or 'crowd' noises. (I get awakened at least once a week by someone calling my name, usually right over my head - it makes me soooo mad because sleep & I aren't exactly best friends).

Basically, the program was adding up the brain activity, muscle paralysis (it's a safety mechanism that prevents people from hurting themselves in their sleep - and sleep walking) rapid eye/ear movement & hypnogogic imagery (pre-dream hallucinations) to explain away people who believe they've been abducted by aliens, but it was still full of good information.