In his book Making Up The Mind, Chris Frith cites research in which experimental subjects are asked to play Tetris for several hours and then report on their dreams. They often report dreams of floating Tetris like blocks. Anyone who has played computer games or stared at spreadsheets for many hours a day will have had a similar experience.
The interesting thing is that even people who have lost the ability to form new memories report the same experience. These are people who suffer from the tragic condition of forgetting everything shortly after they experience it. You can introduce yourself to them every day and they will always think that they have never met you before. But they will dream of Tetris even though they may say that they have never heard of the game.
This is interesting because it reminds us that, as neuroscience increasingly shows, much of what goes on in our brains is not directly accessible to our consciousness and, even if it accessible, it is not necessarily under our conscious control.
Such knowledge is both fascinating, because it reveals ourselves to ourselves, and disconcerting, because it shows once again that we are the sum of our physical brains.