MELANOPSIN, a recently identified protein, plays a key role in a completely new light detection system in the eye, according to researchers from Imperial College London, and American universities, Johns Hopkins University and Brown University.

Professor Russell Foster of Imperial College London at Charing Cross Hospital explained: "It has long been assumed that the rod and cone cells of the retina are responsible for all light detection.

"However, during the last few years, our research has revealed a third light detection system that has lain undiscovered for more than 100 years."

A deeper understanding of these latest photoreceptors may provide new ways to overcome jet-lag and treatments for disorders such as SAD (Seasonally Affective Disorder) that can be caused by a lack of light, particularly during the winter months.

Dr Rob Lucas of Imperial College London at Charing Cross Hospital added: "Melanopsin is particularly important in the detection of bright light as we believe this new photoreceptor is responsible for telling our bodies that it is daytime - daylight is always bright light.

"Apart from regulating pupil size, the melanopsin photoreceptors may be responsible for a broad range of responses to light, including its ability to keep us awake and alert. It is also likely that these photoreceptors are responsible for resetting our internal body clocks to local time following a flight across time-zones."

Melanopsin , expressed in a small number of ganglion cells in the retina of the eye, was tested by measuring light-induced papillary constriction in genetically modified mice that lacked melanopsin.

When mice that lacked melanopsin were exposed to low light, their pupillary response was the same as normal mice, but when they were exposed to bright light, their pupil constriction was incomplete.

The research, entitled: "Diminshed Pupillary Light Reflex at High Irradiances in Melanopsin - Knockout Mice", was published this month in Science.