I'm with R41 -- 40 was the beginning of being middle-aged.. But my 50's have shown me it really is the beginning of the end. I had an sports injury @ 50 that made it difficult for me to exercise, and I put on 20 lbs. The knee surgery I had to try to correct it only left things worse.
According to the National Institute of Neurological Disorders and Stroke (NINDS), the sudden, involuntary jerking of a muscle or group of muscles while drifting off to sleep is called myoclonus. It is caused by sudden muscle contractions, also known as positive myoclonus, or muscle relaxation, which is referred to as negative myoclonus.
I Have Had Sleep Paralysis. Sleep Paralysis Can Lead To Sudden Unexplained Nocturnal Death Syndrome (sunds) a SUNDS victim produces no body movement even though he experiences a myocardial infarction and strong breathing difficulties and should straggle in agony. The death may be caused by the extreme muscle atonia during Sleep Paralysis, which is so severe that even the cardiac muscles and the diaghragm paralyze.