TL;DR: If you don't go into this book expecting it to be a creepy thriller, you will probably love it. The premise is awesome, but for me the suspense/thriller element fell short of expectations. 
 I read this book hoping for something kinda creepy to get me in the mood for Halloween. It was marketed as the big teen thriller for Fall, so I thought for sure that would be what it was. 
 Not so much. 
 That's not to say the book wasn't good, it was. I just wouldn't really classify it as a creepy thriller. And I am a huge pansy, so a book doesn't have to be very scary before I get creeped out. Hell, books that are not even remotely supposed to be scary sometimes creep me out. It is not hard to do. So I feel pretty confident in saying this book definitely falls short of its "perfect for Halloween" marketing strategy. 
 That said, the premise of Nightfall is still really pretty cool. It is set on an island where the sun rises and sets every 28 years, meaning the island gets 14 years of daylight followed by 14 years of darkness. When the sun sets, the people of the island sail to a desert in the south where they spend the next 14 years waiting for the sun to rise on their island once more. Before they leave, however, they strictly follow a very strange set of traditions, all which heavily imply that they are preparing the town for someone else to inhabit during the night. And when Marin, her brother Kana, and their friend Line get left behind, they have the misfortune of finding out exactly what it is that takes over their island after nightfall. 
 The characters are interesting enough. Neither Marin nor Kana are particularly eager to leave the island- Marin because she is going to be cloistered with a bunch of other girls she's never met when she gets there, and Kana because he is blind in the sun, but for some reason regains his sight in the dark. Line on the other hand is the only parental guardian his younger brother has left, which you would think would lead him to make more responsible decisions then going on a romp through the woods hours before they absolutely had to leave or be left to die, but hey that's just me. 
 For a standalone novel, which I'm assuming this is, the world building is quite good. The culture of the islanders, and even of the southerners in the desert who are never directly shown, is portrayed well without any clunky info drops. And the fact that the desert traditions Marin was so desperately hoping to avoid ended up playing a roll in their survival was, I thought, really cool and well executed. 
 I don't want to go too much into what exactly it is that takes over the island after nightfall, because most of the suspense of the novel is based around not knowing what is hunting them. At the same time, I feel like it is in the suspense that this book really falls short. The entire time they are being hunted by these mysterious creatures that apparently live in their town after nightfall, and all the tension is built around what will happen if they are caught, and after all that there is only one major climactic event and, for something relying so heavily on suspense, the climax was a tad disappointing. I don't know what it is I wish would have happened instead, but something about it felt empty. That being said, I am notoriously difficult to impress when it comes to suspense novels, so this might just be me being high maintenance. 
 I also felt like there was a bit left to be desired in terms of character relationships and development. One of the characters has a pretty big bomb dropped on them near the end, while it is acknowledged that it has a huge impact on them very little is done with it, as the novel ends shortly after. On top of that all of the major changes in the relationships between the three characters take place one right after the other near the end of the book, so there is no real time to see the mindsets changing or developing; they just sort of do. Felt a bit rushed to me, but I suppose in life or death situations you either get over it or you don't. 
 All in all it is a good book, but it isn't really anything special. The admittedly awesome premise is enough to carry it to the point that I definitely wanted to keep reading it to find out what happened, but nothing really gives it that "wow" factor that I always really expect out of a thriller. But again, I've said similar things about thriller/suspense novels that other people have loved specifically FOR their wow factor (Girl on the Train, Pretty Girls), so you really just have to read it for yourself.