My best guess is it is a form of epilepsy commonly called "negative myoclonus". Essentially it causes involuntary limb movement whenever you relax. When sleep deprived, it can also happen for me at night before bed when I'm relaxed sitting in a chair. I also noticed my startle reflex is greatly exaggerated during these times. Strangely it ONLY exists when I'm sleep deprived. Right now, for example, I have zero symptoms of this.

https://pubmed.ncbi.nlm.nih.gov/8891393/#:~:text=Negative%20....