What causes these things to happen in your body?

Sometimes, when I close my eyes, I can still see things like little particles floating in water or pieces of dust floating around, when I close my eyes. They appear whitish orange in a black background as my eyes are closed. Does anyone else see that and what is it that I’m seeing?
Also, when I’m snoozing, I sometimes have this sudden sensation of falling down a hole and a spasm occurs, totally waking me up. What the hell is that? What is going on in the body&mind then?