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.
Instead, hypnic jerks seem to be a sign that the motor system can still exert some control over the body as sleep paralysis begins to take over. Rather than having a single âsleep-wakeâ switch in the brain for controlling our sleep (i.e. ON at night, OFF during the day), we have two opposing systems balanced against each other that go through a daily dance, where each has to wrest control from the other.