Life on earth has evolved under a consistent cycle of light and darkness caused by the earth's rotation around its axis. This has led to a 24-hour circadian system in most organisms, ranging all the way from fungi to humans. With the advent of electric light in the 19th century, cycles of light and darkness have drastically changed. Shift workers and others exposed to high levels of light at night are at increased risk of health problems, including metabolic syndrome, depression, sleep disorders, dementia, heart disease, and cancer. This book will describe how the circadian system regulates physiology and behavior and consider the important health repercussions of chronic disruption of the circadian system in our increasingly lit world. The research summarized here will interest students in psychology, biology, neuroscience, immunology, medicine, and ecology.