Rensselaer Research ReviewWinter 2008
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 New Approaches Shed Light on Ways Circadian Disruption Affect Human Health
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Rensselaer researchers made an important discovery about, and came up with a new way to study, the circadian system.

Like a wristwatch that needs to be wound daily for accurate time-telling, the human circadian system — the biological cycles that repeat approximately every 24 hours — requires daily light exposure to the eye’s retina to remain synchronized with the solar day. In a new study published in the June issue of Neuroscience Letters, researchers in the Lighting Research Center (LRC) have demonstrated that when it comes to the circadian system, not all light exposure is created equal.

LRC researchers have also created a small, head-mounted device, called the Daysimeter, to measure an individual’s daily rest and activity patterns, as well as exposure to circadian light — short-wavelength light, particularly natural light from the blue sky, that stimulates the circadian system.

Spectral Opponency

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 New Approaches Shed Light on Ways Circadian Disruption Affect Human Health
Researchers demonstrated spectral opponency by exposing subjects to varyng units of blue light in one eye and green light in the other. They then measured each individual’s melatonin levels, a natural indicator of the circadian clock.
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Short-wavelength light, including natural light from the blue sky, is highly effective at stimulating the circadian system. Exposure to other wavelengths — and thus colors — of light may necessitate longer exposure times or require higher exposure levels to be as effective at “winding the watch.” 

In some instances, exposure to multiple wavelengths (colors) of light simultaneously can result in less total stimulation to the circadian system than would result if either color were viewed separately, a phenomenon known as “spectral opponency.” The LRC scientists have shown that the circadian system shares neurons in the retina — which exhibit spectral opponency and form the foundation for our perception of color — with the visual system. Thus, in principle, the circadian system may be able to distinguish between lights of different colors. 

The findings have profound implications for exploring how lighting can be used to adjust our bodies’ clocks, and they could redefine the way lighting is manufactured, according to Mariana Figueiro, lead author of the paper and assistant professor in the LRC.  

More Than Meets the Eye

To demonstrate that the circadian system exhibited spectral opponency formed in the retina, the researchers 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 and then measured each individual’s melatonin levels, a natural indicator of the circadian clock.

“The first two conditions — exposure to a single color in each eye — did not result in a significant difference in melatonin suppression, while the third condition — exposure to both colors in both eyes — resulted in significantly less melatonin suppression,” said Figueiro. “Even though the amount of light at the eye was the same in all three conditions, when the two colors of light were combined in the same eye, the response of the system was reduced due to spectral opponent mechanisms formed in the retina.”

This indicates that spectral opponency is a fundamental characteristic of how the human retina converts light into neural signals in the human circadian system, according to Figueiro.

Figueiro conducted her research with LRC Director Mark Rea, and Senior Research Scientist Andrew Bierman, who are coauthors on the paper. 

Graphic of the circadian clock adapted from image from Creative Commons.

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“New Approaches Shed Light on Ways Circadian Disruption Affect Human Health”
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