Leaving the nursery lights on at night might be a comfort to your baby or toddler, but could predispose him or her to short-sightedness in later life, according to research published in the 13 May issue of the science magazine Nature

From a poll of parents of children attending a paediatric ophthalmology clinic as out-patients, Richard A. Stone of the Scheie Eye Institute at the University of Pennsylvania School of Medicine, Philadelphia and colleagues found a strong relationship between short-sightedness, or myopia, and exposure to light at night in infants between birth and two years of age.

Only 10 per cent of infants who slept in darkened nurseries became myopic later on in childhood. This contrasted with a figure of 34 per cent of infants sleeping with a 'night light', and a remarkable 55 per cent of infants who regularly slept with the room lights on.

The researchers admit that finding a correlation between nocturnal illumination and subsequent myopia is not the same thing as isolating a cause, and that they have not ruled out other factors.

For example, children sleeping in brightly lit nurseries might become myopic because their parents are myopic - and myopic parents might prefer to leave the lights on so they can see where they are going and don't bump into the crib by accident.

Doing research by questionnaire, rather than direct investigation, also introduces a fuzziness factor: different nurseries are lit in different ways. The researchers were not able to study eye development in standardized, laboratory conditions. However, the figures seem dramatic enough for the researchers to counsel prudence - they advise parents to switch off the nursery lights off as a precaution, while the findings are studied in greater depth.

The research was prompted by two lines of investigation. First, experiments on chicks and monkeys indicate that the growth of the eyes after birth is shaped by the eyes' own experiences, such as their exposure to light, rather than directives from the brain or by growth of the nervous system in general. In terms of anatomy and development, the eyes are outgrowths of the brain. In particular, it is known that the growth of the eyes in chicks is influenced by day length: the more light an eye experiences, the larger it grows.

The problem is that excessive eye growth leads to myopia. If the volume of the eyeball is large in proportion to the cornea and the lens, images become focused not on the retina, but in front of it. This phenomenon is particularly marked for images of distant objects - and the result is myopia.

The second finding prompting the research is the increasing prevalence of myopia, particularly in Asian countries where it approaches 70-90% of the population. However, of the 479 children whose habits were recorded by Stone and colleagues, less than one per cent were of Asian ethnic origin. Most (around 70%) were Caucasian, and almost all the rest were African-American: monitoring the emergence of myopia in children of Asian background is a matter of priority among those interested in unravelling the still largely unknown causes of myopia in the human population.