Several scientific studies have determined that light on the eye’s retina is the primary synchronizer of human circadian rhythms, the biological cycles that repeat approximately every 24 hours. Both the visual system and the circadian system respond to light as it is processed through photoreceptors in the retina. Similar light sources can have similar effects on each system, but recent research demonstrates that similar light sources can also affect each system very differently.
Nocturnal melatonin, a hormone, is used as a marker for the circadian clock, with high levels at night when a person is in a dark environment and low levels during the day with or without light. Researchers at the LRC are working to better understand and quantify light as a stimulus for the circadian system, and they have demonstrated that more light will sometimes be less effective at stimulating the circadian system, as measured by nocturnal melatonin suppression by light.
The power of blue
Blue sky is a mixture of wavelengths dominated by short wavelength light that gives a blue visual sensation. According to Mark Rea, Ph.D., LRC director, the circadian system is essentially a blue sky detector.
“Blue sky is ideal for stimulating the circadian system because it’s the right color and intensity, and it’s ‘on’ at the correct time for the right duration—the entire day,” said Rea.
Exposure to other light colors, as well as exposure to white light, can stimulate the circadian system, but it may take longer to get the desired response and the intensity required may cause visual discomfort, according to Mariana Figueiro, Ph.D., LRC program director.
An LRC research team led by Figueiro has been working to demonstrate the effectiveness of blue light and white light on the circadian system.
Subadditivity occurs when the result of one unit (x) plus another unit (y) does not equal x + y. Instead, the result is a unit less than the sum of its parts. Recently, Figueiro’s team directly tested subadditivity in the human circadian system, the hypothesis being that middle-wavelength (yellow) light actually detracts from the effectiveness of short wavelength (blue) light on suppressing nocturnal melatonin production. The team’s research detailing this phenomenon, known as spectral opponency, is currently available on PubMed and was published in the October 2005 edition of Neuroendocrinology Letters, released in January 2006.
In the study, adult men were exposed for 60 minutes to white light, and again to that same light source with yellow light filtered out. Both conditions exposed the subjects to the same amount of blue light. The testing was conducted during nighttime conditions while scientists monitored the men’s levels of melatonin while exposed to the different light settings. The researchers found that nocturnal melatonin suppression was greater when the yellow light was filtered out. Thus, more light was less effective on the circadian system, supporting the hypothesis that spectral opponency is a fundamental characteristic of how the human retina converts light into neural signals in the human circadian system.
Based on this research and knowledge about the neurophysiology and neuroanatomy of the retina, LRC researchers developed a model to serve as the foundation for a new quantification system of light for the circadian system. “We can now calculate ‘circadian efficacy’ of different light sources,” says Figueiro. “This has profound implications for exploring how lighting can affect human health.”
The impact of blue light on the circadian system was demonstrated in an earlier study led by Figueiro in which the LRC studied the effects of blue-light treatment on older adults, including those with Alzheimer’s disease, at a skilled nursing facility in upstate New York.
In the four-week study, residents suffering from sleep disturbances were exposed to tabletop LED luminaires for two hours every day from 4:30 p.m. to 6:30 p.m.
In the two-phase experiment, groups were exposed to blue LEDs for a two-week period and then red LEDs for a two-week period. The red condition was introduced as a placebo control, because while the circadian system responds best to blue light, it is essentially non-responsive to long-wavelength radiation (red light), according to Figueiro.
Over the course of the experimental light treatment, the research team analyzed the percentage of time the subjects slept between midnight and 6 a.m. The study showed statistically significant increases in sleep after blue-light treatment during this period for all subjects.
Materials provided by Lighting Research Center, Rensselaer Polytechnic Institute. Note: Content may be edited for style and length.
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