I have experienced this occasionally.

When I'm on the verge of falling asleep, but still just a little aware that I'm awake, I have this feeling that the bed/the world below me is turning on me so that my bed, which should be horizontal, starts to slowly move towards a vertical position like the hands of a clock. After a moment, it gets to the point where I feel like I'm about to slide off my bed feet first. However, before I "slide off" the bed, my body jerks, I'm awake, and everything is normal again.

This is a weird feeling, and startles me a little bit every time it happens, but I've never really put too much thought into it; I've always just ignored it and went back to sleep. Has anyone else experienced this before?