The normal and abnormal variations in conscious state operate through three fairly well-understood physiological processes: activation (A), input-output gating (I), and modulation (M). This chapter provides an account of the phenomenology of the variations in conscious state, and shows how three mediating brain processes, activation, input-output gating, and modulation, interact over time so as to account for those variations in a unified way. It focuses on variations in consciousness during the sleep-wake cycle across species and draws on evidence from lesion, electrophysiological, and functional neuroimaging studies. By studying the way that consciousness is normally altered when we fall asleep and when we dream, it is possible to obtain insights about how the brain mediates consciousness. Armed with the AIM model, it is possible to obtain a unified view of the genesis of a wide variety of normal and abnormal changes in conscious experience.