How come I see colors and patterns flashing and swirling when I close my eyes?

Every time I close my eyes, it seems as if there are colors and patterns going across the back of my eye lids. They tend to be circular, almost like a circular fractal design. Sometimes I see square patterns. It's almost as if the images are movies burned into my retinas. I always thought when you closed your eyes you were supposed to see darkness or at least partial shade, but all I see is light bursts.