Light is the paramount environmental signal for the entrainment of endogenous circadian rhythms. Its non‑visual effects, mediated by the retina, exert a profound control over human sleep, mood and systemic physiological homeostasis. Beyond its canonical function in image formation, the retina operates as a primary irradiance detector through a specialized class of neurons, the intrinsically photosensitive retinal ganglion cells (ipRGCs), which utilize the photopigment melanopsin. These cells convey environmental light information directly to the suprachiasmatic nucleus (SCN), the brain's master circadian pacemaker, thereby synchronizing the body's internal timekeeping with the external solar cycle. Compelling evidence demonstrates that the spectral quality of light, particularly within the short‑wavelength blue range, potently modulates neuroendocrine and neural systems via the ipRGC pathway, governing melatonin synthesis, the architecture of the sleep‑wake cycle and affective regulation. The modern light environment, characterized by ubiquitous artificial light at night and pathological states of light perception resulting from ophthalmic diseases such as glaucoma and retinal degenerations, can severely disrupt this synchronization. The consequent circadian misalignment is a significant etiological factor in sleep disorders, depressive symptoms and other systemic morbidities. The retina's integral position within the light‑rhythm‑behavior axis is thus a critical nexus between the visual system and systemic physiology. In addition, the present study outlined the nitric oxide‑cyclic GMP signaling axis in SCN as a critical mediator of photic entrainment. This review provided an in‑depth analysis from an ophthalmic perspective, synthesizing evidence from animal models and human studies to dissect the complex molecular, cellular and network‑level mechanisms of retinal circadian regulation, explore how aberrant photic signaling impacts sleep and mood and critically evaluate the potential of targeted interventions such as light therapy and spectral management in the context of rhythm‑related disorders.
