Body vibrations when falling asleep or waking?

After many google searches for the reason I feel vibration in my head, shoulder and arm just before falling asleep and waking, I found nothing that satisfactorily explained it. Of course anxiety seems to be a common explanation but there was nothing specific enough to help me. I am currently reading Incognito by David Eagleman. It discusses the functioning of the brain as an organ. This morning, going through my routine to get ready for work, I thought of a way to understand the vibrations. Similar to the hypnic jerks we experience occasionally just before we fall asleep, I think the vibrations are intended to arouse me to stay vigilant. Which is the fight or flight mechanism in action. So there is the anxiety connection. Originating in the amygdala. I was thinking Parkinson's or MS but my symptoms don't indicate those conditions, thankfully. It seems counterintuitive to be anxious and sleeping at the same time but the brain can do that. The cerebral cortex sleeps but the life support systems of the brain must stay online, obviously. If you thought you were in danger, sleeping would put you at risk but at some point sleep becomes absolutely necessary. I think the vibrations I have been experiencing are caused by my sympathetic nervous system and not a neurologic disease. Thankfully !