nightlights

The use of nightlights has been the subject of minor controversy and confusion recently. They deserve at least brief discussion.

First off, why use a nightlight?

So that parents can visually check on their baby without turning on a light in the hallway or the baby's room and potentially awakening their child unnecessarily.
So that when Baby awakens from a dream, especially a frightening one, he can rapidly get oriented to his surroundings and realize he is no longer dreaming.

What was all the fuss about? A study¹ was published in the journal Nature which claimed an association of nearsightedness (myopia) in human children with parental use of nightlights. Now the idea of the study was not without some scientific basis. Researchers began to investigate possible links between myopia and ambient lighting after noticing that perpetual lighting on poultry farms affects eye growth in chicks.

The research team asked the parents of 479 children attending an ophthalmology clinic to complete a questionnaire detailing each child's light exposure from birth to 2 years of age. They said they found a dose-dependent relationship of nighttime light levels to myopia. That is, the more light at night, the more nearsightedness observed.

sleeping in total darkness was reported to have a 10% incidence of nearsightedness
having a nightlight on raised this to 35%
for children who slept with a lamp or the room lights on (75 out of 479), the incidence was 55%

Criticisms of the study are legion:

First, the obvious critcism of sample selection bias: these were kids with eye problems by definition - they were at the eye clinic already. This is not a representative sample of anything except kids with poor vision. Drawing any conclusions from it at all is perilous at best.
Next, the study is "retrospective." Parents were asked around age eight what had gone on years earlier. Such data is notoriously unreliable.
There was no quantification of ambient room light attempted beyond nightlight yes or no, or room lighting. There is no mention that a researcher even made any measurements of room light levels in actual households for comparison.
No proposed mechanism was advanced. Classic myopia is caused when the eyeball is out of round - it is too deep or long in relation to its height. How the diffence in photons striking the retina between
- eyes closed in a totally dark room
- eyes closed in a dimly lit room
might conceivable change the shape of the eye is "problematic."
Beware the fallacy of post hoc ergo propter hoc - "after this, therefore because of this." There may be an association, but that definitely does not establish cause and effect. Association is not causation!
Nearsightedness is known to be familial. Perhaps nearsighted parents are both more likely to use a nightlight or lamp to help themselves navigate in the night and also more likely to have nearsighted offsprings on a genetic basis.
Why do we only now notice this really striking effect? Has myopia spread across the globe like a plague in the last hundred years, a dreadful effect of the adoption of electric lighting?

Where are we now? Another study² was published in Nature in the early part of 2000 which found contrary results to the first study, namely that the risk of nearsightedness is not higher in children who sleep in rooms with night lights. A study of 1,220 school children found that

20% of children who slept with no night light were nearsighted
17% of those who slept with a night light were nearsighted
22% of those who slept in a fully lit room were nearsighted

So my personal advice is that parents may continue to use nightlights if they choose until there is more convincing evidence that their use is harmful. Currently available data simply is not convincing that parents should change their practices.

1. Nature 1999;399:112-113
2. Nature 404(6773):143-144, 2000.

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