Civilization has changed the natural light-dark cycle that humans experience. Buildings shield us from the weather as well as the bright daytime sky. Electric light sources not only provide illumination at night and throughout building interiors, they also provide self-luminous displays such as televisions and computer monitors. Epidemiologists and other medical researchers have expressed concern over electric lighting as a potential disruptor of the natural light-dark cycle; a wide range of maladies from insomnia to breast cancer have been statistically associated with disruption of the natural 24-hour light-dark cycle.
Considering the significance of the light-dark cycle for regulating biological functions, and the accumulation of evidence from epidemiological and animal studies linking circadian disruption to compromised health and well-being, it is surprising that so little has been done to quantify light and dark in industrialized societies as they might affect the human circadian system. Given this paucity of photometric data, it is perhaps not surprising that so little has been done to parametrically study the impact of circadian disruption on health and well-being in people. However, without proper photometric data it is essentially impossible to draw valid inferences about the impact of lighting, both natural and fabricated, on human health and well-being.
Concerns have been raised by an advocacy group, the International Dark Sky Association (IDA), over light at night as it affects human health through stimulation of the circadian system. The IDA has drawn attention to the relative spectral composition of different outdoor light sources as a possible concern for human health. In response, the Lighting Research Center and ASSIST sought to provide a quantitative analysis of the impact of light at night, particularly from streetlights of different spectral power distributions, on the human circadian system. Described below is a summary of the analysis methods and results. (For complete details, download the full technical paper.)
Estimates of irradiance levels at the cornea from two commercially available “cool-white” LEDs, a sodium-scandium metal halide (MH) lamp, and a high-pressure sodium (HPS) lamp were considered for three different conditions: a reference condition similar to what has been employed in controlled laboratory conditions, and two practical scenarios that could occur with an outdoor lighting installation. From those irradiances, and assuming a one-hour exposure with natural pupils, it was possible to estimate the degree to which the circadian system of a 20-year old would be stimulated, defined operationally as percentage of nocturnal melatonin suppression.
Reference condition and two lighting scenarios used to calculate effective circadian light stimulation for four light sources.
Melatonin suppression (%) by the human circadian system in response to two “cool-white” LEDs, metal halide (MH), and high-pressure sodium (HPS) sources plotted for a wide range of corneal photopic illuminance levels.
A reasonable and conservative working threshold for suppressing nocturnal melatonin by light at night following a 30-minute exposure would be about 30 lx at the eye for a “white” light source. This working threshold is based upon the determination of a reliable degree of light-induced nocturnal melatonin suppression of 15% or greater. As shown in the figure above,
any given threshold value (10% for this analysis) will show that different light sources, depending upon their spectral irradiance distributions, will require different photopic illuminance levels to be considered above or below that threshold value.
Download the full paper:
Related Publications and News
Light at Night – Special notice published by the LRC’s Light & Health program
Does Architectural Lighting Contribute to Breast Cancer? – Scientific paper published in the Journal of Carcinogenesis
Project Summary Sheet (2010)