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.
What causes you to moan in your sleep? Answer Wiki. When you are in REM sleep, aka the dream part of sleep, your muscles stiffen up as if to keep you from re-enacting your dreams. Therefore, you may be yelling, or speaking in your dreams, but in reality, your muscles cant move to form the words, which results in a sort of moaning sound.