It was a gripping thriller/mystery. The details are rapidly slipping away, but I remember that there was a very dramatic twist involving the main female character: she was not what she seemed to be in a very dramatic way, a hallucination or an illusion of some sort. I remember thinking that although the twist had completely ambushed me, I should have seen it coming, because it made perfect sense.
I wish I'd retained more details. I wish I'd stayed asleep until the end.