Maybe It's Better To Stay in the Dark

Might exposure to artificial light at night increase a man's risk for prostate cancer? An unusual international study indicates that it might, providing provocative new evidence that disrupting the body's natural rhythms may play a role in one of the most common malignancies to afflict men.

The first-of-its-kind study, which involved analyzing satellite measurements of nighttime light emissions and cancer rates in 164 countries, found that nations that emit the most light at night tend to have the highest prostate cancer rates.

While the findings do not prove a cause-and-effect relationship, they follow earlier studies that found men who work the graveyard shift appear to have a higher risk of prostate cancer.

"This study is a very important new piece of evidence," said Richard G. Stevens of the University of Connecticut Health Center in Farmington, who helped the conduct the study. It appears in the current issue of the journal Chronobiology International. "The fact that we are seeing a strong relationship lends plausibility to a real cause-and-effect."

Although more research is needed to confirm the relationship, Stevens and others say enough evidence has accumulated to consider taking steps to reduce unnecessary exposure to light at night.

"There is certainly no need for panic," said George C. Brainard, director of Thomas Jefferson University's Light Research Program in Philadelphia, who was not involved in the new research. "But since there's concern and a growing weight of evidence, prudent avoidance would be advisable."

How light might increase the risk for prostate and other cancers remains unclear. But a key factor may be the hormone melatonin. Melatonin plays an important role in regulating the body's natural rhythms, falling during the day and rising at night. But when people are exposed to light at night, it interferes with that pattern, causing melatonin to plummet. In addition to regulating the body clock, melatonin suppression may affect levels of other hormones, such as testosterone and estrogen, which can fuel prostate and breast cancer. Melatonin suppression may also suppress the immune system.

Women who have jobs that expose them to light at night -- such as shift workers or flight attendants -- appear to have higher breast cancer rates, and a handful of studies have produced similar findings for men and prostate cancer.

Laboratory studies have also found that human breast and prostate cancer cells grow more slowly when exposed to melatonin in petri dishes, and human breast and prostate tumors implanted in rats grow faster when the animals' melatonin is suppressed by light exposure.

"Put this all together and it argues pretty strongly that light-induced melatonin suppression may be at the heart of this," said David Blask, a professor of structural and cellular biology at Tulane University.

In fact, enough evidence has accumulated that the World Health Organization's International Agency for Research on Cancer concluded in 2007 that shift work was "probably carcinogenic," with the strongest evidence being for breast and possibly prostate cancer.

"This new study fits in with that," said Aaron Blair of the National Cancer Institute, who helped prepare the agency's evaluation.