Given the clinical, methodological, conceptual and modelling challenges within the field of sleep medicine, polysomnography (PSG) has emerged as a central diagnostic tool over time. It has been highly beneficial to clinical practice over the years, thanks to the scientific data that it provides. More recently, sleep medicine has sought answers in precision medicine, broadening its quest for biomarkers that take into account environmental factors, big data, and nosological refinement. However, despite these innovative developments that are relatively independent of PSG, sleep medicine remains intimately associated with the latter. The aim of this paper was to show the central role of PSG for sleep medicine. Indeed, PSG is central to sleep medicine, not only due to the empirical data it provides but also because it represents an obligatory passage point (OPP) within the discipline. It crystallizes debate, pulls disparate types of data together, and facilitates a common language for the different specialties involved in sleep medicine, thereby lending legitimacy and credibility to the specialty. Thus, the role of polysomnography as an OPP in the field of sleep medicine reinforces the discipline, especially because critics (e.g., of the Apnea Hypopnea Index) cannot easily find fault with it.