Body’s internal clock is set by newly discovered light detection system in the eye

Many of the body’s responses to large changes in environmental light are controlled by a newly discovered light detection system in the eye, scientists report today.

Researchers from the UK, Canada, the USA and Germany reveal that the eye’s brightness detection system helps set the body’s internal clock, regulate general activity levels and control the size of the pupil.

The new brightness detector system is based upon a molecule sensitive to blue light called opsin, which is quite different from the opsin proteins used in the rods and cones of the eye. The team also found that melanopsin, a recently identified protein, is critical for this brightness detector system to work.

The findings, which are reported online in Nature, may further our understanding of how sleep, energy levels, and emotional state are regulated, and may help develop new treatments for conditions such as jet lag and Seasonal Affected Disorder (SAD).

Professor Russell Foster of Imperial College London, and author on the study said:

“After more than a decade of research we can now be sure that the eye contains two very different light detecting systems. Rods and cones cells that give us our sense of visual space, and this new system, which provides information about the overall brightness of light within the environment.

“When we first suggested that there might be this other light detecting system in the eye we were regarded as heretics. But, you can understand why – for the past 100 years the rods and cones were thought to be the only light sensing cells within the eye, so you can imagine how pleased we are with these new findings.

“We knew something very interesting was going on from our earlier studies on mice with naturally occurring genetic diseases of the eye. These mice lacked rod and cone cells making them classically blind but they could still regulate their biological rhythms and some other responses to light, including changes in pupil size.”

The previous collaboration with colleagues in the USA had also shown that switching off the melanopsin gene upsets the body clock and reduces pupil responsiveness to light, but does not abolish them completely.

Now, the researchers wanted to confirm whether there could be any other unknown light detection systems had produced these results. Using mice that lacked rod, cone and melanopsin function they set out to investigate whether there was any residual response to light. It was shown that while these mice has an intact retina there was no response.

“Our results indicate we have now identified all the light processing systems in the eye. The big challenge ahead is to understand the biochemistry that drives the melanopsin photoreceptors, and how the “classical” and novel photoreceptors systems interact. Essentially we are dealing with a completely unexplored light detecting system of the eye that is likely to have a major impact on many areas of human health and performance,” said Professor Foster.

“Brightness detection system works very different to how light used by the eye to generate an image of our world. The body needs a clock to help it anticipate the different demands of activity and rest.

“Jet-lag is the classic example – where body time and local time get confused. It is only after a few days of exposure to the local light environment that body time and local time become synchronised once again. We now know that this synchronisation is largely dependent upon these blue light sensitive melanopsin photoreceptors,” he added.