Cat having violent dreams

I've a 16 year old male cat, Oliver. When he sleeps, deeply, he starts thrashing about, leg spasms too. I have to really, really wake him as once in that deep state he takes awhile to become alert. His eyes are back in his head if I try to open them. He's the sweetest guy, ever yet he's gently attacked me when waking him. Anyone have advice? The vet said no issues.