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.
View full size image. A hypnagogic jerk is an involuntary muscle spasm that occurs as a person is drifting off to sleep. The phenomenon is so named in reference to the hypnogogic state â the transitional period between wakefulness and sleep. Hypnagogic jerks are also commonly known as hypnic jerks or sleep starts. The muscle spasms may occur spontaneously or may be induced by sound, light or other external stimuli.