Repeated exposure to low doses of light induces retinal damage in vivo in a wavelength-dependent manner

The exposure of the general population to artificial light at night has dramatically increased in recent decades. Current standards for domestic lighting are based on acute exposure to light and consider blue wavelengths to be responsible for phototoxicity. However, meta-analyses pointed out the role of lifelong light exposure in the onset of age-related macular degeneration, suggesting a cumulative effect of light exposure. Here, we investigate the retinal phototoxicity of a repeated exposure to light emitting diodes of different spectral compositions in 6-week-old albino rats. Rats were exposed twice a day for 15 days to retinal doses that were safe in acute exposure (0.1 and 0.2 J/cm² for blue and white lights, 0.2 J/cm² for green light and 0.05 J/cm² for red light). We show that rats repeatedly exposed to blue and white lights display irreversible retinal damage, characterized by a degradation of the global retinal structure, a significant photoreceptor loss, and an increase of stress and inflammation markers. We highlight the role of green wavelengths in the phototoxicity of white light and show the protective effect of the addition of red light to mitigate the phototoxicity of blue light. All of this points out the need to rethink the current phototoxicity standards by taking into account the cumulative effect of the exposure to light and the role of the different parts of the emission spectrum.