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The City Dark

Premiere Date: July 5, 2012

'The City Dark' in Context

Effects of Light on Human Health

In The City Dark, epidemiologist Richard Stevens of the University of Connecticut suggests a link between light at night and breast cancer.

In The City Dark, epidemiologist Richard Stevens of the University of Connecticut suggests a link between light at night and breast cancer. In 1987, Stevens wrote in the American Journal of Epidemiology that "the use of electric lighting, resulting in lighted nights, may produce circadian disruption," which causes changes in hormone levels in the body.

Human beings (and nearly all living things) have evolved under a natural rhythm of day and night. The 24-hour cycle during which the body goes through physiological changes is called a "circadian rhythm," or colloquially the "internal body clock."

Circadian rhythms respond significantly to light and darkness. In fact, melatonin is often called the "hormone of darkness" because it is secreted in the dark and it is believed to play an important role in fighting off cancer. If bodies are consistently exposed to artificial light at night, or LAN — whether from streetlights or a television glare — melatonin hormone production can be disrupted.

Stevens' LAN research has included studies indicating that night-shift workers are almost twice as likely to develop breast cancer as day-shift workers, and the World Health Organization has classified shift work as a probable cause and risk factor for cancer. Although the research linking LAN to increased risk of cancers is still hypothetical, many chronobiologists suggest limiting one's prolonged exposure to bright lights during nighttime hours.

According to the National Institutes of Health, a change in a person's internal body clock impairs his or her ability to sleep and wake at appropriate times and leads to a decrease in cognitive and motor skills. Disruption of these rhythms can contribute to insomnia, depression and cardiovascular disease. However, the Lighting Research Center emphasizes that stimulation of the human circadian system at night is not necessarily a health risk and that the type of light is a major factor, as are the intensity and the duration of light exposure of the retina.

On June 15, 2009, the American Medical Association voted unanimously to adopt resolutions to reduce light pollution and glare, advocating for use of energy efficient, fully shielded outdoor lighting.

Photo caption: Light pollution in a NYC bedroom.   Credit: Wicked Delicate Films LLC.

» International Dark-Sky Association. "Light Pollution and Human Health."
» Tala, Angelina. "Lack of Sleep, Light at Night Can Raise Cancer Risk." Medical Daily, October 11, 2011.

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