I've noticed that sometimes (not very often, maybe once every few months or so) I will have a very strange type of dream, with absolutely no visual content, just thought. It's rather like being awake and just having my eyes closed and thinking about stuff, however because I'm dreaming my thought processes are very different. This seems to happen more when I'm studying for exams or working on something that requires a lot of logica/analytical thought.

It's extremely hard to explain, but it's like the thoughts become tangible, like objects that I can touch, but there's no visual content, everything's pretty much black. Sometimes I come up with some interesting concepts that never occurred to me before, and I remember them when I wake up. I'd be interested to hear if anyone else experiences dreams like this. I'm almost tempted to think that maybe they're dreams that happen outside of REM (but I don't know if this is possible?)