Lucid dreams. There are particular dreams in which you can understand that you are sleeping at the current moment. It is called - "Lucid dreams". But if you say like this, it's like you say nothing about it. In those dreams you can understand who are you and where are you came from, and you can remember your purposes. It is very interesting state. You can do something you cannot do in the reality and more else things. But if you have nothing to do in this kind of dreams, you can easily get enjoyment by stay there in short time.