Brief light exposure at night disrupts the circadian rhythms in eye growth and choroidal thickness in chicks.

Changes in ocular growth that lead to myopia or hyperopia are associated with alterations in the circadian rhythms in eye growth, choroidal thickness and intraocular pressure in animal models of emmetropization. Recent studies have shown that light at night has deleterious effects on human health, acting via "circadian disruptions" of various diurnal rhythms, including changes in phase or amplitude. The purpose of this study was to determine the effects of brief, 2-h episodes of light in the middle of the night on the rhythms in axial length and choroidal thickness, and whether these alter eye growth and refractive error in the chick model of myopia. Starting at 2 weeks of age, birds received 2 h of light between 12:00 am and 2:00 am for 7 days (n = 12; total hours of light: 14 h). Age-matched controls had a continuous dark night (n = 14; 14L/10D). Ocular dimensions were measured using high-frequency A-scan ultrasonography on the first day of the experiment, and again on day 7, at 6-h intervals, starting at noon (12 pm, 6 pm, 12 am, 6 am, 12 pm). Measurements during the night were done under a photographic safe-light. These data were used to determine rhythm parameters of phase and amplitude. 2 groups of birds, both experimental (light at night) and control, were measured with ultrasound at various intervals over the course of 4 weeks to determine growth rates. Refractive errors were measured in 6 experimental and 6 control birds at the end of 2 weeks. Eyes of birds in a normal L/D cycle showed sinusoidal 24-h period diurnal rhythms in axial length and choroid thickness. Light in the middle of the night caused changes in both the rhythms in axial length and choroidal thickness, such that neither could be fit to a sine function having a period of 24 h. Light caused an acute, transient stimulation in ocular growth rate in the subsequent 6-h period (12 am-6 am), that may be responsible for the increased growth rate seen 4 weeks later, and the more myopic refractive error. It also abolished the increase in choroidal thickness that normally occurs between 6 pm and 12 am. We conclude that light at night alters the rhythms in axial length and choroidal thickness in an animal model of eye growth, and that these circadian disruptions might lead to the development of ametropias. These results have implications for the use of light during the night in children.