Given Example described an observational study in which it was found that children who slept with a nightlight or in a fully lit room before the age of 2 were more likely to be nearsighted than children who slept in darkness.

Does this mean that sleeping with a light on as an infant causes nearsightedness? What are some other possible explanations?

Example
Lighting the Way to Nearsightedness A survey of 479 children found that those who had slept with a nightlight or in a fully lit room before the age of 2 had a higher incidence of nearsightedness (myopia) later in childhood (Sacramento Bee, May 13, 1999, pp. A1, A18).