The Age of Miracles by Karen Thompson Walker

Middle school and soccer start as usual for Julia but then many things begin to shift.  Friendships change, she begins to see boys differently, but more importantly, the earth itself begins to rotate more lowly.  Minutes are gained in each day, then hours.  Days get longer and hotter, evenings longer and colder.  People divide into those who stick with a 24 hour "day" dark or light and others known as "real-timers" follow the rhythms of sunrise and sunset.  As days and nights stretch longer and longer, the structures of society begin to fail.  Science finds ever more costly ways to cope but can do nothing to stop the slowing.  A strange sickness tied to the slowing begins to affect many people and rumors of government conspiracy abound.  It all begs the question, if the end seemed inevitable, how would you choose to live?