Sleep medicine and dream science have long been dominated by empirical approaches. Volume 2 of this handbook highlights emerging theories of sleep disorders and the role of social and environmental factors in sleep health, showing how conceptual models guide effective application of empirical work. Featuring contributions from leading experts, chapters examine mechanisms of sleep regulation, functions of sleep and dreaming, and present diverse frameworks side by side, outlining core assumptions, mechanisms, and implications before addressing deeper complexities. Designed for students, researchers, and clinicians across sleep science, neuroscience, psychology, biology, and medicine, the handbook invites critical engagement with models and reflection on how they shape contemporary understanding and research.