In this book, some are day people and some are night people. They are set into that category and cannot be out during the wrong time. People are fighting against it. Everything is pretty locked down. There are enforcers everywhere and medical procedures affecting some. Of course, a boy and girl from the two different societies fall for each other. You know it's inevitable, but what I didn't expect is that everything doesn't go perfectly for them. I like that bit, makes it seem more real.