Although it is widely recognized that there are significant variations in the circulation throughout the 24-hour day, these patterns have often been conceived as nuisance variables rather than functional differences. Responses of the circulation are examined here not as stable phenomena, but as integrated functions highly sensitive to environmental conditions, and driven by internal and external pacesetters. In this volume, cardiologists with a primary interest in hypertension or heart disease, chronobiologists, and psychologists join together for the first time to give the reader the opportunity to learn the extent and clinical significance of these changes from a chronobiological, behavioral and physiological perspective.