The Effect of Light Intensity and the Timing of Light Exposure on Choroidal Thickness

To investigate the short-term effects of different light intensities on choroidal thickness and determine if these responses vary with time of day and refractive error.

Twenty young adults (10 myopes, 10 emmetropes; mean age 29 ± 4 years; mean left-eye spherical equivalent -2.53 ± 1.84 and -0.12 ± 0.20 D, respectively, were exposed to three lighting conditions: bright light (2000 lx), normal room light (350 lx) and darkness (<0.1 lx) for 60 min in a randomised order on 6 separate days, in the morning (09:00-11:00 h) and evening (17:00-19:00 h), with at least 24 h between visits and all sessions completed within a 1-month period. Choroidal thickness of the left eye was measured at baseline, during 60 min of light exposure and 30 min post-exposure, to assess recovery.

Exposure to 2000 and <0.1 lx induced significant subfoveal choroidal thickening during the exposure period, peaking at 60 min (bright: +16 ± 4 μm; dark: +13 ± 6 μm; both p < 0.05), with values returning to baseline after 30 min of recovery. No significant changes occurred under 350 lx (p = 0.17). The greatest thickening was at the fovea (+16 ± 5 μm bright; +13 ± 6 μm dark) and parafovea (+13 ± 7 μm bright; +9 ± 6 μm dark), with smaller perifoveal changes. Bright light induced greater thickening in the evening than in the morning (p = 0.004), while darkness showed no time-of-day effect (p = 0.69). No significant refractive group differences were found (p = 0.90).

One hour of bright light or darkness caused transient choroidal thickening in young adults. Changes were most prominent in the foveal region, and bright light produced stronger evening responses. These findings demonstrate that short-term choroidal thickness is dynamically modulated by both light intensity and time of day.