In this UK sample there was no evidence of an association between night lights and myopia. A similar conclusion was drawn from studies in the United States by Gwiazda et al11
and Zadnik et al
However, in subjects from Singapore and China, Saw and co-workers13
did find a weak association between night lights and myopia that almost reached statistical significance, and in a very recent study, Loman et al17
found a significant association between myopia and the number of hours of complete darkness to which young adults were currently
exposed at night.
The population studied here had a higher prevalence of myopia (64%) and a lower prevalence of hyperopia than the general population (see distribution of offspring mean spheres in Fig 1) suggesting a source of ascertainment bias. As has been noted previously,11
given that the subjects studied by Quinn et al1
were aged between 2–16 years, they showed a similar high prevalence of myopia (with 30% of subjects myopic). Although a myopia prevalence of 64% is typical of university students,17
our study was also likely to have been affected by response bias, with more myopes choosing to participate than non-myopes. Such selection and response biases have the potential to either inflate or mask the effect being investigated.18,19
By selecting subjects attending a university paediatric ophthalmology clinic, the population studied by Quinn et al5
is likely to have suffered from a different source of selection bias, but possibly a similar response bias.
In contrast with the lack of an effect from night lights, this study once again confirms the higher prevalence of myopia in Asians compared to Europeans, and the influence of parental myopia in determining the refractive errors of their children.1
However, the heritability estimate obtained here (0.31) is much lower than that obtained in two careful twin studies that were carried out recently3,4
both of which used considerably more subjects than were included in the present investigation (n=506 and 114 twin pairs, respectively). We speculate that several factors might have contributed to this difference. Firstly, our heritability has wide confidence intervals and thus could be a considerable underestimate (and while the linear regression residuals for the calculation do not show a significant deviation from normality, the sample is too small to provide a robust test of this). Secondly, our subjects had a more varied ethnicity, and a higher proportion of high myopes, compared to those examined in the two twin studies. Thirdly, our heritability estimate is subject to a source of bias, since emmetropic parents would be less likely to wear spectacles, and thus would have been excluded from our analysis. Finally, because twin studies make the assumption that “common environment” effects are independent of zygosity, they tend to overestimate heritability in comparison with population studies.20
In conclusion, our results suggest that night-time light exposure played a lesser part than genetic factors in the myopia development of this UK student population.