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Troy, N.Y. -  6/30/2010

New ASSIST technical paper evaluates the impact of outdoor lighting on the human circadian system

Outdoor LightThe Alliance for Solid-State Illumination and Technologies (ASSIST) recently published a technical paper that quantitatively evaluates the impact that outdoor lighting has on the human circadian system. A growing interest in the role that light plays on human health, combined with the increasing use of white outdoor lighting with high correlated color temperatures and a short-wavelength spectral component, has prompted some advocacy groups to raise alarms about the potential impact of outdoor lighting on human health. This technical paper shows that this concern may be unfounded under realistic scenarios; the impact of streetlights and other outdoor lights may have only a small, if any, impact on human biological cycles.

Every species on earth exhibits circadian rhythms – biological cycles that repeat approximately every 24 hours. These cycles are usually in synch with the natural light/dark cycle, with the human circadian system being most sensitive to and activated by short-wavelength (blue) light. Civilization has created environments where 24-hour light/dark patterns can potentially be disrupted, from occupying deep core buildings to bright electric illumination lighting interiors at night. Disruption of these light/dark patterns, which can occur with regular exposure darkness during the day and bright light at night, has been associated with breast cancer, insomnia, obesity, and a wide range of maladies. 
To determine whether and how much outdoor lighting systems might stimulate and potentially disrupt the human circadian system, researchers at the Lighting Research Center (LRC) investigated the impact that realistic outdoor lighting scenarios might have on these biological cycles.
Assuming a published model of human circadian phototransduction previously developed by LRC researchers, investigators for this study evaluated four typical outdoor light sources: two commercially available “cool-white” LEDs, a sodium-scandium metal halide (MH) lamp, and a high-pressure sodium (HPS) lamp. LRC investigators evaluated these light sources under three conditions: a controlled laboratory condition as a reference and two typical streetlight scenarios for distance and viewing angle outdoors. The latter two viewing conditions represented real-life conditions that people could actually experience as they come into contact with outdoor lighting. Assuming a one-hour exposure with natural pupils for a 20-year-old, LRC researchers calculated the percentage of melatonin suppression that would occur from exposure to each light source under each condition. “We determined whether there was enough light reaching the retina to stimulate the circadian system as measured by melatonin suppression,” said lead researcher Mark Rea, director of the LRC. Melatonin is a common biological marker used in the study of circadian rhythms.
Results showed that under the two practical street lighting conditions, three of the four light sources would not meaningfully stimulate the human circadian system after one hour of exposure. One source (a 6900 K LED) is predicted to provide 3–10% melatonin suppression.  According to the LRC, a reasonable and conservative working threshold for suppressing nocturnal melatonin by light at night would be 30 lx at the eye from a “white” light source for 30 minutes.  This working threshold is based upon the minimum exposure to "white" light that could produce reliable nocturnal melatonin suppression of 15% or more.
 “Although stimulation of the circadian system is not necessarily synonymous with health risk, it is essential to determine if and to what degree light sources used outdoors at night might stimulate the circadian system. This study is a good start toward quantitatively understanding if outdoor lighting poses a concern,” said Rea. However, Rea strongly cautioned that further study is still needed to fully understand the causal link between light at night and human health and to understand how the circadian system may or may not be stimulated by light exposure. 
More information, including a link to the paper, can be found here:
ASSIST is a collaboration between researchers, manufacturers, and government organizations. Its goal is to identify and reduce major technical hurdles currently facing solid-state lighting. The Lighting Research Center conducts research, demonstration, and educational activities on behalf of ASSIST. ASSIST is sponsored by Acuity Brands Lighting; Bridgelux; China Solid State Lighting Alliance; Cree; Everlight Electronics Co., Ltd.; Federal Aviation Administration; GE Lumination; ITRI, Industrial Technology Research Institute; LG Innotek; Lighting Science Group; Lite-On; NeoPac Lighting; New York State Energy Research and Development Authority (NYSERDA); OSRAM SYLVANIA/OSRAM Opto Semiconductors; Permlight; Philips; Sharp Laboratories of America; Seoul Semiconductor; United States Environmental Protection Agency; WAC Lighting.

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.