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 re-purposed 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. Rhonda Brown is Associate Professor of Health Psychology in the Research School of Psychology at the Australian National University, Australia. With national and international collaborators, she examines predictive relationships between stress, affective distress, sleep, fatigue, and illness outcomes in patients and community-well individuals. Over the last 20 years, she has also examined work-stress, burnout, communication performance, and empathy in medical staff and medical and psychology students; as well as impaired immunity and infection comorbidity in anorexia nervosa patients. Einar Thorsteinsson is Associate Professor at the University of New England, Australia. He worked at La Trobe University in a fire fighting decision making lab for two years before he moved back to focus on health psychology at the University of New England where he has built national and international research collaborations covering areas such as stress, burnout, sleep, social support, depression, anxiety, adolescent coping and health, and psychological wellbeing.