This book explores the interplay between physiological and psychological states in light of increasing evidence that they exert subtle, long-term influences not only on mood, but also perception, judgement, and cognitive processes in general; these, in turn, affect behaviour. Drawing on his own data from subjective assessments of mood and research by others, the author addresses questions such as what determines a person's mood and its changes; what is the relationship between mood and sugar snacking, smoking, coffee drinking, late-night worry, depression, and insomnia; what effect do exercise, time of day, nutrition, and sleep have on mood. This book will be of interest to researchers in personality, clinical, and physiological psychology and to laypersons interested in the topic.