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.
Alcohol intoxication and drug abuse can also trigger seizures while sleeping. Nocturnal seizure can occur as a result of alcohol or drug withdrawal. Brain infection such as meningitis can also trigger episodes of seizure while sleeping. They can also occur during daytime. Tumor in the brain.