Light therapy encompasses a broad spectrum of interventions, ranging from established clinical applications to emerging and experimental indications. This review evaluates the current state of light therapy across medical disciplines, emphasizing the need to distinguish validated clinical uses, exploratory findings, and preclinical research. The biological effects of light therapy are diverse, with mechanisms depending on wavelength, route of delivery, and biological target. Ocular exposure to bright light primarily acts through the circadian system to influence mood and sleep, whereas peripheral photobiomodulation (PBM) and ultraviolet (UV) phototherapy act directly on local tissues to promote healing. Mitochondrial activation is a proposed general mechanism, but its centrality is likely modality- and context-dependent. Robust clinical evidence supports bright light therapy for seasonal affective disorder (SAD) and UV-B for psoriasis, while moderate evidence exists for dermatological and sleep applications. For delirium, bright light therapy has shown promise in small-scale studies, but large-scale confirmatory clinical trials are still lacking. Other reported uses, including pain management, ischemia-reperfusion (IR) injury, clotting disorders, and infectious lung injury, are largely supported by preclinical and mechanistic studies, particularly those involving intense light that elicits the circadian core protein Period 2 (PER2). The clinical relevance of these mechanisms remains under investigation. Overall, light therapy remains safe when used according to established protocols. This review aims to provide a nuanced synthesis of the field, highlighting both the promise and the limitations of light therapy and emphasizing the importance of the evidence hierarchy in guiding clinical translation and research priorities.
