This book introduces a new theory on the substantial comorbidity that exists between many illnesses and disorders and concurrent symptoms such as pain, impaired sleep and fatigue. The specific illnesses and disorders discussed include obesity, diabetes mellitus type-II, medical illnesses including cardiovascular disease and sleep-disordered breathing, insomnia, disordered eating such as binge-eating disorder and night-eating syndrome, affective distress (anxiety and depression), and comorbidities that are linked to eating disorders such as anorexia nervosa and bulimia nervosa. The book posits that the comorbidities are the result of a complex bio-psycho-behavioral mechanism that includes circadian rhythm dysfunction. It examines the statistical and methodological (e.g. measurement) problems that can complicate the understanding of comorbidity and explores a broad range of novel, existing, and repurposed therapy approaches that could have utility in treating comorbid disorders. This book will be of great value to academics as well as practitioners working in the field of psychiatry, health psychology and medicine more broadly.