Some psychologists have said that the tune brings forth the lyric in the sub-conscious that relates to something you are thinking about at the moment. If you can chase down the lyric to that tune you might find an idea that is "buried" that will surface.

If you are a practicing musician and are learning a tune, and in your waking hours you go over it many times, it will follow you throughout the day and in the night. It actually may be a valid form of practicing the tune until you get it.