The idea of writing this book came to me 6 years ago, when I started teaching an upper-level course on circadian rhythms and was unable to find a suitable textbook. The available books were either too limited in scope (addressing specialized topics in the field) or too old and out-of-date. Teaching and research responsibilities prevented me from writing the book until very recently, when I took a year off from academia and found enough time to go over the extensive literature in the field and organize my own thoughts.