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.
Sleep starts are also known as hypnic or hypnagogic jerks. They are sudden, brief and strong contractions of the body or one or more body segments. They occur as you are falling asleep. Sleep starts usually consist of one strong jerk that affects much of the body. The arms and legs are most likely to be affected. A sharp cry may also occur.