The human circadian system - the biological cycles that repeat approximately every 24 hours - requires daily light (preferably natural or daylight) exposure to the eye's retina to remain synchronized to the solar day. In a study published in the June issue of Neuroscience Letters, researchers demonstrated under the controls for studying our circadian system that not all light exposure is the same.
According to Mariana Figueiro, lead author of the paper and assistant professor at the Lighting Research Center (LRC) at Rensselaer Polytechnic Institute, these findings have significant implications for examining the way lighting can be used to adjust the human biological clock, and redefine how lighting is manufactured.
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 stimulating the circadian response. In fact, sometimes exposure to multiple wavelengths (colors) of light simultaneously can result in less total stimulation to the circadian system than would result if each color were viewed separately. This color response is called "spectral opponency." LRC scientists have shown that the circadian system shares retinal neurons, which exhibit spectral opponency and form the foundation for the perception of color within the human visual system. Hence, in theory, the circadian system may be able to distinguish between lights of different colors.
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 results indicated that more exposure to more colors in each eye had a significant lack of melatonin suppression. This response of the system was reduced due to spectral opponent mechanisms formed in the retina. This is an important finding, as it indicates that nocturnal melatonin, a hormone produced at night under conditions of darkness, is used as a biochemical marker for the circadian clock. Scientific evidence suggests that disruption of this system - and thus the melatonin cycle - may result in increased malignant tumor growth, as well as poor sleep quality, lack of alertness, seasonal depression, and immune deficiencies. Melatonin and the circadian clock are thought to impact many other heath conditions. Studies will continue to reveal new ways that light and light control may be used in healthcare.
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