Interesting. I noticed that when I have a really good night's sleep (I mean, proper 8-9 hours with no interruptions) I don't dream at all. Or at least I don't remember them.

On the other hand, when there's something wrong, I have dreams that are literally designed to wake me up. I had one recently about Nazis occupying my city and cleaning out houses one by one, while I was watching through the window. Man.