Degree of light damage to the retina varies with time of day of bright light exposure.

After adaptation to cyclic illumination (14:10 light-dark cycle; intensity = 1.4 log ftc) male albino rats were exposed to brighter light (2.15 log ftc) seven hours daily for 25 days. The extent of retinal damage was determined histologically. Daily exposure to bright light in the morning produced greater photoreceptor loss than daily afternoon exposure. Severe retinal damage was partially prevented by brief light adaptation (20 minutes duration, at 1.4 log ftc intensity) before the onset of morning exposure. However, light adaptation did not remove the differential effect of time of day. The authors conclude that increased light exposure during the period of rod outer segment phagocytosis enhances photoreceptor damage.