Scientists have found how to make eye cells sensitive to light, opening new ways to treat some forms of blindness.
Eye checks can spot diseases
Experts at Imperial College London teamed up with colleagues at the University of Manchester to study a protein, melanopsin.
Activating melanopsin in cells that do not normally use it made them sensitive to light, they told Nature.
The discovery might also help treat people who get depressed as the nights draw in.
The back of the human eye, called the retina, contains cells known as photoreceptors that interpret light levels to allow us to see.
Much human blindness is due to diseases of the retina, such as retinitis pigmentosa and macular degeneration, in which the photoreceptors are destroyed.
Currently there is no cure for such diseases, and once sight is lost it cannot be recovered.
Until recently, experts had thought there were only two types of photoreceptors - rods and cones.
Seeing the light
But experiments on mice which have had their rods and cones destroyed, reveals that other cells in the retina also have some form of light response.
Scientists have suspected that melanopsin is important to all of these 'light sensitive' cells.
The London-Manchester team set out to study melanopsin in more detail.
In mice, they found turning on a gene for melanopsin caused nerve cells to work like photoreceptors.
Although making cells in the eye responsive to light is not a cure for blindness, the researchers are working with engineers to develop prosthetic retinas that might help people with sight disorders to see more clearly.
Professor Ron Douglas of London's City University, said: "Much effort is being put into both retinal transplants and even electronic light-sensitive implants.
"However, both approaches are a long way from being clinically effective and may never be so.
"The current research suggests another possible line of therapy."
He said it could be that melanopsin genes could be inserted into intact cells in otherwise diseased retinas, turning them into functional photoreceptors.
But it was extremely unlikely that it would ever lead to fully restored sight.
"The resulting 'vision' may well be little more than black and white light sensitivity, but it would be a start," Prof Douglas said.
Professor Chris Inglehearn, professor of molecular ophthalmology at Leeds University, said: "This is highly significant. They are getting at the primary process of what makes a cell sensitive to light."
He said it could also have broader implications than just eyes.
"It could be important for sleep, insomnia, depression and seasonal affective disorder too."
Melanopsin has been implicated in conditions such as these linked to disruptions of the body's internal clock and daily rhythms.