A systematic examination of sleep and dreaming that blends physiological theory with psychological analysis. The text treats sleep as a partial suspension of external sensation and voluntary control while vital functions continue, evaluates competing accounts of cerebral blood flow, and describes bodily and mental signs that accompany falling asleep. It then investigates the material and neural mechanisms behind dream imagery, surveys the varieties and characteristics of dreams, explores their psychological interpretation, and exposes common misconceptions, concluding with tentative hypotheses and an appeal for more detailed observational study.