December 16, 2025
By Ashley Wallace Tucker, OD, FAAO, FSLS
Photo Credit: Getty Images
This IMI review presents current evidence on how light influences refractive development and myopia, drawing from both animal research and human studies. The paper begins by highlighting one of the most consistent findings in myopia epidemiology: children who spend more time outdoors have a significantly lower risk of becoming myopic. Large-scale prevention initiatives in Singapore, Taiwan and China, many recommending about two hours per day outdoors, have demonstrated reductions in myopia incidence and, in some cases, slowed community-level declines in visual acuity. However, the mechanisms behind this protective effect are not fully understood.
Light Intensity and the Dopamine Hypothesis
Animal studies across species (chickens, guinea pigs, mice, tree shrews and rhesus monkeys) show that bright light exposure inhibits myopic eye growth, particularly in form-deprivation myopia. Even brief daily exposure (two to six hours) to light intensities between 2,500 and 40,000 lux reduces axial elongation in an intensity-dependent manner. The proposed mechanism centers on retinal dopamine: bright light increases dopamine release, which appears to act as a “brake” on excessive eye growth. Pharmacologic blockade of dopamine eliminates the protective effect of bright light, underscoring its role in growth regulation. Yet dopamine likely does not encode the direction of growth; rather, it serves as a permissive signal allowing other cues to guide emmetropization.
Human data also hint at a possible dopamine connection: children treated with methylphenidate hydrochloride (a dopamine reuptake inhibitor) show lower myopia incidence or slower progression in preliminary studies.
Spectral Composition (Color) and Emmetropization
Beyond intensity, the spectral properties of light, its color composition, also influence ocular growth. Because of longitudinal chromatic aberration (short wavelengths focusing anterior to long wavelengths), the retina may use differences in cone activation as a cue for defocus. Altering chromatic input via narrow-band lighting often leads to refractive errors across species. Short-wavelength (blue) light can be protective in chicks and mice, whereas long-wavelength (red) light has protective effects in primates and tree shrews. These species-specific differences caution against direct translation to humans.
Importantly, all “white light” is not equal. The paper shows (via spectral plots) that sunlight has a broad, continuous spectrum, while LEDs, fluorescents and halogen lamps have narrow or spiked distributions that produce distinct opsin activation patterns even when illuminance (lux) is matched. This means two light sources with the same lux value may stimulate the retina very differently.
Human Evidence: What We Know and Don’t Know
Human studies offer supportive but incomplete evidence for a causal role of bright light:
- Increasing classroom illumination (e.g., to ~550 lux) reduces myopia onset and axial elongation in school children.
- Wearable-sensor studies show slower axial growth in children and young adults exposed to higher average daily light levels.
- Seasonal variation (slower progression in summer) supports intensity-linked effects.
However, the field lacks evidence-based thresholds for protective intensity, duration, spectral characteristics, or timing of light exposure.
Conclusions and Future Needs
Although increasing outdoor time is a validated public-health recommendation for delaying myopia onset, the current evidence does not yet justify clinical guidelines for artificial light-based therapies. The authors call for better study standardization and reports not only on light intensity, but also the radiant power across individual wavelengths, allowing meaningful comparisons between different sources.
Abstract
IMI: The Role of Light in Refractive Development and Myopia: Evidence from Animal and Human Studies
Regan Ashby; Elise N. Harb; Lisa A. Ostrin; Daniel Ian Flitcroft; Frank Schaeffel; Cindy Karouta; Timothy Gawne; Ranjay Chakraborty; Kate Thomson; Scott Read; Rigmor C. Baraas; Raymond P. Najjar; Kathryn Rose; John R. Phillips; Pei-Chang Wu; Xiangui He; Xiao Nicole Liu; Weizhong Lan; Barbara Swiatczak; Hidemasa Torii; Audrey Chia; Ian Morgan
Spending time outdoors is consistently associated with delayed myopia onset in children and has been incorporated into prevention programs. While the underlying mechanisms remain under investigation, substantial evidence supports sunlight exposure as a key contributing factor. This review evaluates the evidence supporting this association. Animal studies demonstrate that light characteristics-such as intensity, chromaticity, and photoperiod-can influence refractive development, often postulated to function through modulation of the dopaminergic system. However, translating these findings to humans is challenging due to limited data. Evidence remains insufficient regarding how specific light properties-including intensity thresholds, exposure durations, spectral composition, and temporal patterns-affect human myopia.
Consequently, although clinical recommendations for outdoor time (e.g., 2 hours daily) are well supported by epidemiological studies and widely endorsed, current literature does not yet support evidence-based guidelines concerning specific characteristics of light exposure. Addressing this gap requires further randomized controlled trials using standardized wearable technologies to better quantify children’s visual environments and identify light-related cues relevant to myopia development. Interest in light-based therapies is growing, but most interventions remain in early stages of development/testing. Due to limited efficacy data or unresolved safety concerns, no clinical recommendations can currently be made. Interpreting light-related research-especially from across animal models-requires caution. Species-specific differences in ocular transmittance and opsin distribution/tuning complicate the translation of chromatically related findings between species and to humans.
Moreover, the term “white light” can be misleading, as artificial sources vary spectrally from each other and from sunlight, resulting in varied patterns of opsin activation. Studies should therefore attempt to report not only light intensity, but also the radiant power emitted at each wavelength to enable meaningful comparisons.
DOI: 10.1167/iovs.66.15.5
