Nocturnal myoclonus is a benign type of muscle contraction that occurs at the onset of sleep. It has recently been recognized to be associated with fibromyalgia. It occurs as a twitch or sometimes a vigorous jerk of the leg or legs. The problem may first be noticed by your sleep partner, but in any case, it interrupts the path to sleep. Myoclonus can be treated with a drug called Klonopin (clonazepam), if necessary.
(Source: Your Personal Guide to Living Well with Fibromyalgia, an official publication of the Arthritis Foundation.)