There are multiple things I do...
READ : Because it's time to sleep, and you're focusing on one word after another, your eyes get tired until you want to close them.
MAKE A STORY IN MY HEAD : I do this pretty much every night, whether I'm tired or not. Come up with a basic plot, characters, names, places, anything you want, until you have a story telling itself inside your head. Just think up what makes sense, or something that doesn't make sense at all.
CONTINUE A STORY: The next night, I just continue one or more stories, and I'll switch to another one at a good stopping point. Think about each story as if it were a movie, but a really descriptive and long movie. So, the description of a book, the look of the movie.
Good luck!