Although this is relaxed in the summer, I have a time set when one must be ready for bed, and then a time after that which is for reading, then a lights-out time.
When I find DC reading late, I typically have a talk about how important getting enough sleep is, and my expectations that one takes care of oneself and of good and cheerful behavior. If I don't see the good and cheerful behavior, the natural consequence is a much earlier bedtime that night.
Typically, I don't have a long term problem. If it goes on another night, I probably would just take the book away. I am known for staying up a bit too late for an "exciting part" of a book from time to time too.
