I can share that several years ago I started noticing (and my husband started seeing) that during my sleep I occasionally jerk my leg, arm, sometimes whole body. During a typical nap, it might happen like 6-10 times. Just peculiar. Occasionally wakes me up. It seems to happen more during a mid afternoon nap than at night. In short, I know this is another side effect of MS, for sure. I’d love to know what is actually triggering this. Is it like a short circuit in my brain, or is it part of a dream. Feels like a jolt of electricity.

What you are having is called a myoclonic jerk, not a spasm. Even healthy people get those, typically when falling asleep. Those are not cause for worry in themselves, though they can be unpleasant. You can injure yourself, say if you bump into something, but a muscle under tonic load doesn't clonic jerk, so they won't make you fall when upright, or drop a heavy object you were carrying (assuming you have no grip issues) Interestingly, mine were much more pronounced when I was on SSRIs (actually, maybe Citalopram specifically) Progressive Muscle Relaxation (or Yoga Nidra if you're more hip) may help - they did for me, but I never had them that bad that I would wake up from them. Also, Cannabis, but that's my answer to nearly everything 🤣 (Though the 'ohshiti'mfalling' myoclonic jerk right as you fall asleep is one many people get - it will be the sole experience many healthy people will have of it - but I wouldn't call that waking up from it.)