Ha, that sounds like an accurate description of me. I always do that when I get bored or space off during class. Also right before I go to bed. Every. Single. Day.

Sometimes I get sick of it, or the images seem to go bland because of how many times I've conjured them up, but I usually get back into it pretty quickly. Personally, I don't think it's a sign of depression, because I don't feel the least bit depressed when I'm thinking about death. It's the opposite, actually. I entertain myself by thinking about death.