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Troy, N.Y. -  6/25/2010

Researchers define light as it impacts the human circadian system

Photo Credit: National Institute of General Medical SciencesIn a paper recently published in the Journal of Circadian Rhythms, researchers at Rensselaer Polytechnic Institute's Lighting Research Center (LRC) 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 used as a practical field device 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) at Rensselaer Polytechnic Institute is the world's leading center for lighting research and education. Established in 1988 by the New York State Energy Research and Development Authority (NYSERDA), the LRC conducts research in light and human health, transportation lighting and safety, solid-state lighting, energy efficiency, and plant health. LRC lighting scientists with multidisciplinary expertise in research, technology, design, and human factors, collaborate with a global network of leading manufacturers and government agencies, developing innovative lighting solutions for projects that range from the Boeing 787 Dreamliner to U.S. Navy submarines to hospital neonatal intensive-care units. In 1990, the LRC became the first university research center to offer graduate degrees in lighting and today, offers a M.S. in lighting and a Ph.D. to educate future leaders in lighting. Learn more at

About Rensselaer Polytechnic Institute
Founded in 1824, Rensselaer Polytechnic Institute is America's first technological research university. Rensselaer encompasses five schools, 32 research centers, more than 145 academic programs, and a dynamic community made up of more than 7,900 students and more than 100,000 living alumni. Rensselaer faculty and alumni include more than 145 National Academy members, six members of the National Inventors Hall of Fame, six National Medal of Technology winners, five National Medal of Science winners, and a Nobel Prize winner in Physics. With nearly 200 years of experience advancing scientific and technological knowledge, Rensselaer remains focused on addressing global challenges with a spirit of ingenuity and collaboration.