Credit: P. Cinzano, F. Falchi (University of Padova), C. D. Elvidge (NOAA
National Geophysical Data Center, Boulder). Copyright Royal Astronomical
Society. Reproduced from the Monthly Notices of the RAS by permission of
Blackwell Science.
Here's an interesting article on a topic we've discussed before: http://www.wsj.com/public/article_print/SB121692767218982013.html
About two-thirds of the world's population, including almost everyone in the continental U.S. and Europe, no longer see a starry sky where they live. For much of the world, it never even gets dark enough for human eyes to adjust to night vision, reported an international team that mapped the geography of night lighting.
As I've grown older I definitely feel the need to be in complete darkness for a number of hours each night. I always thought this was a psychological need, but perhaps there's more to it?
Last month, Harvard Medical School epidemiologist Eva Schernhammer and her colleagues reported that nurses who regularly worked the night shift had a higher incidence of colorectal cancer than women who only worked daylight hours. Writing in the Journal of the National Cancer Institute, Dr. Schernhammer concluded that working a night shift in rotation at least three nights a month for 15 years or more may increase a woman's risk of colorectal cancer.
In January, Itai Kloog at Haifa University in Israel and researchers at the University of Connecticut reported in Chronobiology International that the incidence of breast cancer among women living in brightly lit neighborhoods was as much as 73% higher than among women in areas where night-time darkness was the norm.
Many researchers believe exposure to artificial light disrupts our nighttime production of the hormone melatonin that, among other things, suppresses tumor development. Even two weeks of intermittent nightly light exposure can seriously curtail melatonin production, studies show.
Such research isn't sufficient proof that darkness keeps us healthy. The findings are persuasive enough, however, that the International Agency for Research on Cancer at the World Health Organization last December added the night shift to its formal list of probable human carcinogens.
There's a nice high res map of night sky brightness here (see low res version above): http://www.lightpollution.it/worldatlas/pages/index.htm