Sometimes, just as I am falling asleep my body will jerk or twitch and cause me to wake up. I know other people who have experienced this as well. What causes this? Is it some sort of muscle spasm?
This is common, and is not a signal of any disease process. It is called Sleep Myoclonus and rarely is so disruptive that it needs evaluation or treatment. The cause is not clearly understood, but reaching the state of deep relaxation tends to cause more sensitivity to a stimulus, causing the jerk.