You sound like you might have myoclonus. I had this symptom..mine was myoclonic jerks that came out of nowhere and jolted the bejesus out of me. This only happened at night; it was one of my many symptoms of vitamin B12 deficiency. And yes you can have symptoms with B12 in "normal" range sad to say. Been there done that! Although eventually over many decades my B12 went below the range. I take sublingual B12 spray daily for my autoimmune pernicious anaemia. This info is from the wikipedia article Myoclonus...They didn't mention B12 deficiency..humph! lol...
"Myoclonus is a brief, involuntary twitching of a muscle or a group of muscles. It describes a medical sign and, generally, is not a diagnosis of a disease. These myoclonic twitches, jerks, or seizures are usually caused by sudden muscle contractions (positive myoclonus) or brief lapses of contraction (negative myoclonus). The most common circumstance under which they occur is while falling asleep (hypnic jerk)."
Myoclonic jerks may occur alone or in sequence, in a pattern or without pattern. They may occur infrequently or many times each minute. Most often, myoclonus is one of several signs in a wide variety of nervous system disorders such as multiple sclerosis, Parkinson's disease, Dystonia, Alzheimer's disease, Gaucher's disease, subacute sclerosing panencephalitis, Creutzfeldt–Jakob disease (CJD), serotonin toxicity, some cases of Huntington's disease, some forms of epilepsy, and occasionally in intracranial hypotension. Some researchers indicate that jerks persistently may even cause early tremors."