By Rob Stein
Washington Post Staff Writer
Tuesday, February 17, 2009
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.
The possible negative effects of too much light at night makes sense given that for most of human history people were exposed to far fewer hours of light, Stevens said.
"Once we got fire, and then the electric light bulb was invented, things really started to change," said Stevens, who published a study last year with colleagues in Israel on the relationship between light at night and breast cancer.
For that study, the researchers analyzed satellite measurements of nighttime light levels in neighborhoods throughout Israel. After taking into account differences in the neighborhoods that might explain variations in cancer rates, such as wealth, ethnicity and average number of children, the researchers found no link between night lighting and lung cancer. But the researchers found that localities with higher night lighting had higher breast cancer rates.
The new research for the first time applies the same technique on an international scale to examine prostate cancer. The association between high nighttime light emissions and high prostate cancer rates remained even after researchers conducted statistical analyses to account for other factors that might explain the findings by examining differences in gross domestic product, electricity consumption and urban population density.
On average, the countries with the highest nighttime light emissions had a prostate rate of about 157 cases per 100,000 men, compared with about 67 per 100,000 among the countries with the lowest nighttime light emissions. The United States, which has among the highest nighttime light emissions, has a prostate cancer rate of about 125 cases per 100,000. Bangladesh, which has the lowest nighttime light emissions, has less than one case per 100,000.
"It's remarkable," Stevens said.
There was no association between light and lung or colon cancer, which strengthens the case because those cancers are not believed to be related to hormone levels.
If a cause-and-effect relationship is proven, a number of steps could be taken to try to minimize the effects of light exposure at night. Light toward the blue spectrum appears to have the most impact on melatonin, so replacing light systems with those that emit light more toward the red end of the spectrum could help, Stevens said. Some communities have started designing street lamps that direct more of their light downward so they shine less in residents' windows, for example, Stevens said.
Stevens and others recommend that individuals take such small steps as making sure they sleep in a fully darkened room and minimize their light exposure if they do wake up in the middle of the night.
"When you go to the bathroom in the middle of the night, don't turn on that big bank of lights on the vanity. We know that's enough to start lowering your melatonin," he said. "Instead, just have a dim red bulb in the bathroom as a night light. It's enough to see, and it won't affect your melatonin."
While Blask and others recommend taking melatonin supplements, Stevens and others recommend against that.
"Some people say, 'Just take a melatonin pill.' But taking a tablet could disrupt your normal circadian rhythm," Stevens said. "What's much better is get an adequate amount of dark time. Rather than taking a pill as a quick fix, try making your lifestyle more consistent with where we came from."