It's your subconscious working on a problem... One example would be scenario where you can't remember some word, and you decide you can't remember it and stop

It's your subconscious working on a problem... One example would be scenario where you can't remember some word, and you decide you can't remember it and stop thinking about it, your brain continues to search for it in the background (subconsciously)... and then the word comes to you like an interrupt. I had the same thing with studying for exams (I would dream of the lecture notes and when I wake up in the morning I can perfectly recall everything). There was a study posted at HN where not completing a task at work, leaves your brain working on it even after you leave your office, only to have the solution pop up in the middle of the night. All of this can be a result of a stress. You need to take a vacation and relax. Recharge your batteries or you'll be feeling drained really soon...