Sleep is one of the most complex and intriguing biological phenomena in various fields of scientific research. It goes beyond a mere decline in consciousness and alertness to perform multiple functions at different levels. This theoretical study comes in the context of growing scientific interest in understanding the mechanisms regulating sleep and its stages, given its pivotal role in maintaining an individual’s physical and psychological health. In this article, we aim to provide a focused theoretical review that clarifies the nature of sleep by reviewing its basic stages, each of which performs a specific function in maintaining physical, neurological, and cognitive balance. We also analyzed the mechanisms regulating sleep, represented by the circadian process and the homeostatic process. To address the research problem of questioning the basic stages of sleep and its precise regulatory mechanisms, we adopted a descriptive-analytical approach based on a review of recent relevant theoretical literature. Based on the above, the importance of this work lies in its contribution to deepening the scientific understanding of sleep as a dynamic and interactive process. Sleep is not an isolated function; rather, it is closely linked to various cognitive, emotional, and immune systems. Any disruption to this system could have serious consequences, not limited to sleep-wake cycles alone, but extending to other vital functions. In other words, based on the premise that sleep is a fundamental brain function, we have attempted to provide precise scientific explanations to understand its nature, stages, and regulatory mechanisms.