The effects of brief high intensity light on ocular growth in chick eyes developing myopia vary with time of day.

2020 
Abstract Evidence suggests that the relevant variable in the anti-myopigenic effect of increased time spent outdoors is the increase in light intensity. Because light is the strongest Zeitgeber, it is plausible that the effects of bright light exposure depend on time of day, and may impact circadian rhythms. In these studies, we asked whether the effects on eye growth rates and ocular rhythms of brief daily exposures to bright light differed depending on time of day in eyes developing myopia in response to form deprivation (FD) or negative lens-induced hyperopic defocus (LENS). We also studied the effects of concurrent exposures to brief hyperopic defocus and bright light. Exp. 1 : Starting at 12d, chicks wearing monocular diffusers or −10 D lenses were exposed to 3 daily hours (hr) of bright light (30K lux) in the morning (FD: n = 12; LENS: n = 7) or evening (FD: n = 21; LENS: n = 7) for 6 daily exposures. Controls wore diffusers or lenses but weren't exposed to bright light (FD: n = 14; LENS: n = 9). Exp. 2 : Untreated chicks were exposed to bright light in the morning (n = 12) or evening (n = 14) for 6 exposures. Controls were not exposed to bright light (n = 11). Exp. 3 : Chicks were exposed to 2 h simultaneous monocular hyperopic defocus and bright light in the morning (n = 11), mid-day (n = 7) or evening (n = 8) for 5 exposures. “Not bright” lens-wearing controls were data from published work (Nickla et al., 2017). High frequency A-scan ultrasonography was done at the start and end to measure growth rates. The FD group in Exp. 1 and the morning and evening groups in Exp. 3 were measured at 6-hr intervals over the final 24 h to determine parameters for the rhythms in axial length and choroidal thickness. Brief bright light in the evening inhibited eye growth in eyes wearing diffusers or lenses relative to controls (X vs C: FD: 316 vs 468 μm; p = 0.026; LENS: 233 vs 438 μm; p = 0.03); morning bright light had no effect. There was no differential effect of time of day of exposures on the rhythm in axial length; for choroid thickness, “time” accounted for the variance between groups (2-way ANOVA interaction p = 0.027). In untreated eyes, bright light in the morning had a small but significant inhibitory effect relative to evening exposures (803 μm vs 679 μm; post-hoc p = 0.048). Eyes exposed to simultaneous hyperopic defocus and bright light were significantly more inhibited relative to “not bright” controls for the morning exposures (interocular differences: −207 vs 69 μm; p
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