10

I've heard this claim many times, including this CNN report from 1999 which claims that "Night-light may lead to nearsightedness".

From the article:

...research in chicks [demonstrated] that the relative proportions of light and dark during the 24-hour day greatly affected eye growth and refractive development.

Even low levels of light can penetrate the eyelids during sleep, keeping the eyes working when they should be at rest. Taking precautions during infancy, when eyes are developing at a rapid pace, may ward off vision trouble later in life.

Although they admit that it may be a correlation, not a causation, the study's senior author still makes the following recommendation:

"Still, it would seem advisable for infants and young children to sleep at night without artificial lighting in the bedroom until further research can evaluate all the implications of our results."

Have there been more recent, more definitive studies to support or debunk this commonly-held belief?

11

Do nightlights during childhood cause myopia later in life?

No, probably not.

This was actually pretty interesting, it's like a go to example of confounding.

So it was originally found that the amount of light correlated with myopia.

Myopia and ambient lighting at night (pdf)

The prevalence of myopia and high myopia during childhood was strongly associated with ambient light exposure during sleep at night in the first two years after birth.

But, follow up research shows no correlation, so what happened?

Vision: Myopia and ambient night-time lighting

Myopia is a common affliction (one in four adult Americans is near-sighted1), and juvenile-onset myopia is believed to be due to a combination of genetic and environmental factors2. Results from animal experiments indicate that light cycles may affect the development of myopia3, 4, and Quinn et al. claim to have extended these to humans5. They reported a strong association between childhood myopia and night-time lighting before the age of two: there were five times more children with myopia among those who slept with room lights on than in those who slept in the dark, and an intermediate number among those sleeping with a dim night-light5. However, we have been unable to find a link between night-time nursery lighting and the development of myopia in a sample of schoolchildren.

It turns out there are two underlying correlations. First myopic parents typically have myopic children. Second, myopic parents are more likely to use night time lighting for their children.

Vision: Myopia and ambient night-time lighting

Quinn et al. report a strong association between myopia in children and their exposure to night-time lighting during their first two years1. We have been unable to confirm this surprising result, but we find that myopic parents are more likely to employ night-time lighting aids for their children. Moreover, there is an association between myopia in parents and their children2, 3.

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