On occasion if I cannot fall asleep in my room, I will lay on my back with my eyes open just thinking. After some time, I see blobs of usually black or greyish color swirl across my visual field, usually until I cannot see anything but the grey or black color in front of my eyes. It is the same as HI but I have my eyes open. What would be causing this and has anyone had similar experiences?

Also... I wasn't sure where to post this so sorry if the thread is out of place.