My boyfriend's sleep shaking?

I have a question that i really hope somebody can help me with. I recently moved in with my bf. I've been noticing lately that he makes a lot of jerky movements in his sleep. It can be anywhere from his hands to his feet or his whole body at the same time. He does this every night and it kind of has me concerned. He has no idea that he even does it. Does anyone know what may be causing this? Any help would be great. Thx alot!