US scientists have found that children who sleep with a light on are significantly more likely than children who sleep in the dark to grow up short-sighted and having to wear glasses.

BBC Health Correspondent James Westhead: "Children's eyesight is getting worse"

In the trials, children under the age of two who slept with a light on were five times more likely to be short-sighted than those who slept in the dark.

Toddlers who slept with a "night-light" were three times more likely to be short-sighted.

Short-sightedness, or myopia, is the inability to focus on distant objects. It is thought to be a risk factor for blindness in later life.

It is caused by excessive growth of the eyeball, which grows particularly quickly before the age of two. Scientists believe light at night may stimulate the eyeball to grow.

The US team questioned the parents of 479 children about whether their sons and daughters, before the age of two, slept with room lighting, a night-light or in darkness.

The children were aged between two and 16 at the time of the study, with an average age of eight.

The researchers found that 10% of children who slept in the dark were short-sighted, but for those who slept with a night-light the number who were short-sighted was 34%, and, for those who slept with a room light on, the figure was 55%.

Risk factor

Professor Richard Stone, from the Scheie Eye Institute at Pennsylvania University in Philadelphia, is part of the research team.

Professor Richard Stone explains the team's work

He said: "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 near-sightedness.

"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."

The researchers, who reported their results in the journal Nature, said their findings might explain why short-sightedness has become more common over the last 200 years.

Ten per cent of children now need to wear glasses.

Early findings

But Professor Stone told BBC Radio 4's Today programme it would be premature to say that light actually made children short-sighted.

BBC Radio 5's Science Specialist Matt McGrath offers his view of the study

"The study shows an association and is the first effort to tackle the problem. But the association is very strong, even if it does not establish a cause and effect."

Many researchers believe that an increase in close-up work, such as reading and writing, may be to blame.

But the authors of the new study speculate that greater ambient night-time levels associated with urban living might contribute to the growing rate of short-sightedness in developing countries.

Previous research on chicks has shown that the relative proportions of light and dark during the 24-hour day greatly affects eye growth and focusing development.

Parents 'should not worry'

Gill Adams, consultant ophthalmic surgeon at Moorfields Eye Hospital in London, urged parents not to worry about night-lights.

Gill Adams says light could be a factor in eye development

She said: "The most important factor in a child later becoming short-sighted is whether or not the parents are short-sighted.

"Environmental factors may play an additional role."

Laura Galbraith, head of child clinical psychology at Fife Primary Care Trust, Scotland, based at Stratheden Hospital, said: "Ideally children should learn to cope with the dark, because, after all, darkness is part of life, and it will help them sleep better.

"We advise parents not to turn the light on when comforting their child because then light becomes associated with security and comfort, and is seen as a positive thing, while dark is seen as a negative thing.

"But we wouldn't force children to put up with the dark if it worries them. It's best to turn the light off gradually, maybe using a dimmer switch."

Other factors

Optician Ellot Most says factors other than sleeping in light might be to blame for poor eyesight.

Elliot Most says children watch too much TV

He said: "The necessity for children to spend a lot of time reading and writing, they are spending more time in front of the television and they are sitting in front of their computer games more, and that is influencing their eyesight."

BBC Radio 5 Science Specialist Matt McGrath said the research did not offer the final word on the subject.

"What the researchers are saying is that, in some respects, it's OK to leave the landing light on and the door open - they're not too concerned about ambient light exposure," he said.