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For 2/3 of the earth’s population, it is never dark enough to see the stars. For a frightening image of how our night skies are lit with artificial light, and the growth in light pollution see The night sky in the World. Light pollution not only consumes excessive energy, but may affect human health. According to the Wall Street Journal, light pollution can disrupt melatonin, a hormone that helps regulate circadian rhythm and may be important in preventing cancers. (“It’s all about the lighting” – July 25, 2008 )

Serum levels of melatonin decrease when humans are exposed to light at night (indoors or outdoors). The impact has been studied primarily in rotating night shift workers, who are at increased risk of certain types of cancers and other chronic diseases.

Postmenopausal women who had worked rotating night shifts for 30 years or more had a moderately elevated risk of breast cancer as compared with those who never worked shifts (Schernhammer et al, 2001). A June 2008 study by the same author found that increased melatonin levels were associated with a statistically significant decrease in the risk of breast cancer in postmenopausal women. In another study, women who worked at least three night shifts a month for 15 years or more were found to be at increased risk of colorectal cancer.(Schernhammer et al, 2003).

Some scientists suggest that everyone should take care to avoid both natural and artificial light during their normal sleeping hours. That includes the bathroom light.

Note: Taking melatonin pills may have side effects and may actually worsen disruption of the circadian cycle.

Canada does not regulate light pollution. Some US states have “dark sky” legislative initiatives .

Be Dark-Sky Friendly!
The International Dark-Sky Association recommends “dark sky friendly” outdoor lighting options.

Find those Dark Skies!
For those who want to see the starry night sky instead of just hearing about it in songs, one option is to join astronomers who gather at Gordon’s Park on Manitoulin Island in early August. Enjoy counting the satellites as they move through the stars!


Cinzano P, Falchi F, Elvidge CD. The first world atlas of the artificial night sky brightness. Mon Not R Astron Soc 2001;328:689-707 (includes a technical definition of lighting pollution).

Schernhammer ES. Berrino F, Krogh V et al. Urinary 6-sulfatoxymelatonin levels and risk of breast cancer in postmenopausal women. J Natl Cancer Inst. 2008 Jun 18;100(12):898-905

Schernhammer ES, Laden F, Speizer, FE et al. Night-shift work and risk of colorectal cancer in the nurse’s health study. J Natl Cancer Inst. 2003 June 4; 95 (11): 825-828

Schernhammer ES, Laden F. Speizer FE, Willett WC, Hunter DJ, Kawachi I et al. Rotating night shifts and risk of breast cancer in women participating in the Nurses’ Health Study. J Natl Cancer Inst. 2001 Ocober 17;93(20):1563-8

Stevens. RG. Artifcial lighting in the industrialized world: circadian disruption
and breast cancer
. Cancer Causes Control (2006) 17:501-507

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