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LRC News

Summer 2010
Brain illustration. Image credit: National Institute of General Medical Sciences
Image credit: National Institute of General Medical Sciences
Circadian Light Defined in Journal Paper

In a paper recently published in the Journal of Circadian Rhythms, LRC researchers have defined light as it impacts the human circadian system.  Circadian rhythms are biological rhythms that repeat approximately every 24 hours. Exposure to the natural sunrise and sunset synchronizes our circadian rhythms to exactly 24 hours. Circadian disruption by irregular light/dark patterns have been associated with increased risk for breast cancer, diabetes, obesity, heart disease, sleep disorders, and other ailments. 

LRC researchers coined the term “circadian light” as spectrally weighted retinal irradiance that stimulates the human circadian system.  The definition of circadian light is based upon the potential for light to suppress melatonin synthesis at night.  Melatonin is a hormone produced at night and under conditions of darkness and is used as a marker of the circadian system. Formally, light is defined in terms of how it stimulates the human visual system, but this limited definition precludes the impact that light has on other biological systems, such as the circadian system.  Because exposures of light and dark on the retina regulate the circadian system, and because circadian disruption has broad health implications, it is important to develop a new definition of light that characterizes the impact light has on this important biological system.

“As the conversation about circadian light evolves,” said LRC Director Mark Rea, “it will become necessary to develop a formal spectral sensitivity function for the circadian system based on the neuro-anatomy and neuro-physiology of the retina.”

The paper, “Circadian Light,” utilizes a discussion of light for human vision to lay a foundation for a definition of circadian light (CL).  Reference is also made to the Daysimeter, a practical field device used to measure personal light exposures in everyday life.  An electronic version of this article can be found online at:


About the Lighting Research Center

The Lighting Research Center (LRC) is part of Rensselaer Polytechnic Institute of Troy, N.Y., and is the leading university-based research center devoted to lighting. The LRC offers the world's premier graduate education in lighting, including one- and two-year master's programs and a Ph.D. program. Since 1988 the LRC has built an international reputation as a reliable source for objective information about lighting technologies, applications, and products. The LRC also provides training programs for government agencies, utilities, contractors, lighting designers, and other lighting professionals. Visit

Contact:lrcnewsletter@rpi.eduPhotos & Graphics:Dennis Guyon
Editor:Dennis GuyonWeb Production: Christine Kingery
Contributing Writers:Christine Kingery, Mary Cimo, Dennis Guyon
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