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.
Generally, if the jerking does not interfere severely with sleep patterns, it is nothing to worry about. Some types of jerking may be a sign of a more serious problem. Some newborn babies may twitch or jerk their limbs in sleep, which can be very frightening for the parents, especially if it is their first born. It is understandable because this jerking may resemble a seizure due to a neurological disorder.