Keeping the nursery lights on at night might keep nervous infants pacified but it also seems to make it more likely they will wear glasses in later life.
A remarkable study by researchers at the University of Pennsylvania Medical Centre and the Children's Hospital of Philadelphia indicates that children who sleep with a light on in their bedrooms at night before the age of two may be at significantly higher risk of developing myopia - shortsightedness.
The parents of 479 children aged from two to 16 were asked whether their children slept with full room lighting, with a night light or in darkness before the age of two. The researchers found that 10 per cent of the children who slept in darkness were myopic at the time of the study.
Of those who slept with a dim night light before age two, 34 per cent were myopic and 55 per cent of those who slept with a full room light on before age two were shortsighted.
"Our findings suggest that the absence of a nightly period of full darkness in early childhood may be an important risk factor in the future development of nearsightedness," writes Dr Richard A. Stone, professor of opthalmology at the university's Scheie Eye Institute and senior author on the study.
"The study does not establish that night-time lighting during early childhood is a direct cause of myopia, and there are undoubtedly other risk factors.
"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," he added.
The eye goes through an important growth phase in the first few years after birth, according to Dr Graham E. Quinn, a paediatric opthalmologist at the Children's Hospital of Philadelphia. "The earliest years of life appear to represent a critical time in the proper development of the eye," he said.
Myopia affects at least one in four people today but it was not always so, the researchers point out. The work may provide a novel explanation for the increasing incidence of myopia over the last two centuries, as populations shifted from agriculture to urban living. Earlier theories held that the myopia was caused by "nearwork", reading and other close-at-hand occupations.
The study was a clinical extension of results from laboratory research in chicks, which showed that the relative proportions of light and dark during the 24-hour day greatly affected eye growth and refractive development.