Body clock can distinguish between different colours of light

Washington, June 7 : The human circadian system seems to have the ability to distinguish between different colours of light, say scientists.

The circadian system is a term used for biological cycles that repeat approximately every 24 hours, and require daily light exposure to the eyes' retina to remain synchronised with the solar day.

Mariana Figueiro of the Lighting Research Center (LRC) at Rensselaer Polytechnic Institute says that short-wavelength light, such as natural light from the blue sky, is highly effective at stimulating the circadian system.

A light of any other wavelength may require longer exposure times and higher exposure levels to be effective, she adds.

It may be recalled that the colour of a light depends upon its wavelength, and changes with the wavelength.

In certain cases, exposure to multiple colours of light simultaneously can result in less total stimulation to the circadian system than would result if either colour were viewed separately, a phenomenon known as "spectral opponency".

Mariana says that her team has discovered that the circadian system shares neurons in the retina that exhibit spectral opponency, and form the foundation for the human perception of colour.

In principle, the circadian system may be able to distinguish between lights of different colours, she says.

During a study, Mariana and her colleagues exposed 10 subjects to three experimental conditions-one unit of blue light to the left eye plus one unit of green light to the right eye; one unit of blue light to the right eye plus one unit of green light to the left eye; and half a unit of blue light plus half a unit of green light to both eyes.

Thereafter, the researchers measured each subject's melatonin levels, a natural indicator of the circadian clock.

"The first two conditions - exposure to a single colour in each eye - did not result in a significant difference in melatonin suppression, while the third condition - exposure to both colours in both eyes - resulted in significantly less melatonin suppression," Figueiro says in a study report, published in the journal Neuroscience Letters.

"Even though the amount of light at the eye was the same in all three conditions, when the two colours of light were combined in the same eye, the response of the system was reduced due to spectral opponent mechanisms formed in the retina," she says.

Mariana says that that observation was an indication that spectral opponency is a fundamental characteristic of how the human retina converts light into neural signals in the human circadian system.

She adds that the findings also verify the accuracy of a new quantification system LRC researchers developed in 2006 to calculate the "circadian efficacy" of different light sources.

According to her, the tool called the model of human circadian phototransduction correctly predicted the circadian system response demonstrated under each of the three experimental conditions.

Mariana has revealed that the model seems to correctly predict the circadian response to any light source, and can be used as the foundation for a new system of circadian photometry, much like the current system of photometry based on human vision.

She insists that quantification of light as a stimulus for the circadian system provide new scientific insights into how the human body processes light for the circadian system.

Her team is now studying the way time of night affects the potency of light exposure.

ANI