I experience this alot when I'm supposed to write down my dreams but I'm too tired and end up falling back asleep. It's a form of dream induction actually, you visualize what you do and if you're just about to fall asleep, the thoughts become more solid and eventually a dream. It's basicly what you do when you WILD, only you do it unintentionally. Try to take advantage of it! I know it can be very hard to relax and not get excited when you're trying consciously, but whenever you find yourself looking at your clock you should always RC. Might lead to a lucid or two

If you have more questions to ask, just ask away. Good luck and keep on dreaming!