High exposure to outdoor lighting at nighttime may be a risk factor for breast cancer development, a new study suggests.

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Researchers suggest that nighttime exposure to outdoor light may raise the risk of breast cancer for some women.

From an analysis of almost 110,000 women, researchers found that those who resided in areas with high levels of outdoor light at nighttime were more likely to develop breast cancer compared with women who lived in areas with low levels of outdoor light during the night.

Lead study author Peter James, of the Harvard Pilgrim Health Care Institute at Harvard Medical School in Boson, MA, and colleagues published their findings today in the journal Environmental Health Perspectives.

After skin cancer, breast cancer is the most common cancer among women in the United States. This year, it is estimated that around 252,710 new cases of invasive breast cancer will be diagnosed in the U.S.

Risk factors for breast cancer include inherited gene mutations, alcohol consumption, overweight or obesity, and lack of exercise. Increasingly, research has suggested that nighttime light exposure may also play a role in breast cancer risk.

Studies have associated exposure to artificial light at night with a reduction in melatonin, which is the hormone that regulates our sleep-wake cycles.

Darkness triggers an increase in melatonin, which tells us when it is time to sleep. As such, when we are exposed to artificial light at night, melatonin levels decrease. This can cause sleep problems, which previous research has linked to increased breast cancer risk.

For the new study, James and his team sought to further explore the link between nighttime light exposure and breast cancer risk. To do so, the researchers analyzed the data of almost 110,000 women, all of whom participated in the Nurses’ Health Study II between 1989 and 2013.

Using data from satellite images, the team assessed the levels of outdoor light at night in the areas each woman resided. Data were also gathered on the women’s health, participation in night shift work, and socioeconomic factors.

Overall, compared with women who lived in areas with the lowest levels of outdoor light at night, those who resided in areas with the highest levels were found to have a 14 percent greater risk of breast cancer.

On further investigation, the researchers found that the increased risk of breast cancer with high levels of outdoor light exposure was only valid for premenopausal women and women who were former or current smokers.

Additionally, this association was found to be strongest for women who worked night shifts. The researchers speculate that this is down to their greater exposure to artificial light at nighttime and the disruption of their sleep-wake cycles.

While further research is needed to confirm these results and the possible mechanisms behind them, James and colleagues believe that their study may have identified another potential risk factor for one of the most common cancers among women.

In our modern industrialized society, artificial lighting is nearly ubiquitous. Our results suggest that this widespread exposure to outdoor lights during nighttime hours could represent a novel risk factor for breast cancer.”

Peter James