For years growing up, I'd notice that sometimes when I had my eyes closed, there were sometimes glowing circles in the darkness. Sometimes they'd be colorful and others they were like the typical photo of an eclipse. It was usually just as I was drifting off, I'd notice them.

Years later I realized that I could make them appear at-will by wiggling my eyes around (a la REM), and that lead me to conclude that it's likely my overly-long (i'm severely myopic) eyeballs physically stimulating my optic nerves.

I don't notice it as much lately, and I avoid doing it on purpose, in case it's not healthy.