For some time now, exposure to light at night and shift work that requires working at night have been linked to increased risk of developing chronic diseases such as heart disease, diabetes and some cancers. A 2010 article in the journal Environmental Health Perspectives noted that “several studies over the last decade have suggested that the modern practice of keeping our bodies exposed to artificial light at night, or LAN, increases cancer risk, especially for cancers (such as breast and prostate cancers) that require hormones to grow.”

See, humans evolved long before the advent of electricity and artificial light. Our ancient ancestors would rise with the sun and head to bed when it got too dark to see, and that pattern has been ingrained into our genes over millennia. But in the past 100 years or so – a split second in evolutionary terms – there’s been a rapid proliferation of nighttime light sources and a shift in how humans conduct themselves, now that we have the power to light up the night and stay awake later. But that man-made ability may be at odds with our ancient physiology because of how light exposure can disrupt circadian rhythms and the secretion of melatonin, a hormone that helps us sleep. It’s now believed that too much light exposure and the disruption it causes to various aspects of our physiology could actually be making us sick.

Recent research has furthered this discussion in terms of breast cancer risk. In an August study in the journal Environmental Health Perspectives, researchers at Harvard University found an association between LAN and breast cancer risk. Using data gathered as part of the Nurses’ Health Study II – a massive cohort of more than 116,000 registered nurses across 14 states who were between the ages of 25 and 42 in 1989 – lead author Peter James, assistant professor in the department of population medicine at the Harvard Medical School, and his team analyzed the correlation between higher levels of nighttime light exposure and breast cancer incidence.

LAN measurements were based on “satellite images that are averaged over years, so they’re stable outdoor lights that are visible at night. That could be traffic or streetlights or lights from buildings or houses,” James says. This study did not look at indoor light sources, such as fluorescent lighting, television or computer screens or cell phones, which emit a blue light that has also been linked with sleep disturbance, disruption of circadian patterns and potentially with chronic diseases such as cancer and diabetes. James says these indoor light sources could contribute to outdoor light at night, “but probably are not a major source because you couldn’t view them from space.”

Once the team had these images of bright nighttime areas, they mapped them over the locations where women who’d had breast cancer lived to determine incidence rate in the context of LAN exposure. They noted higher rates of breast cancer in areas with higher levels of LAN.

Women who had the highest levels of LAN exposure had an estimated 14 percent higher risk of developing breast cancer compared to women with the lowest exposure to LAN. James says they aren’t certain why this link exists, but have theorized that “light at night disrupts circadian patterns by decreasing your nocturnal secretion of melatonin,” the sleep-inducing hormone. There also appeared to be a dose-response link, meaning that the more LAN a woman was exposed to, the higher her chances were of developing breast cancer.

These findings were noted in premenopausal women only, James says, which could be because “the cohort is primarily premenopausal women, just based on the age of the participants.” He says the team may not have had enough data from post-menopausal women to make a clear association between LAN exposure and breast cancer risk among post-menopausal women.

In addition, the observed increase in breast cancer risk seemed to be confined to estrogen-receptive-positive breast cancers, meaning cancers that use the hormone estrogen to grow. This is significant because “melatonin can potentially influence your estrogen regulation. And that’s the theorized pathway towards breast cancer risk,” James says. If your melatonin levels are out of whack, that could impact your estrogen levels, which in turn could lead to the development of breast cancer.

The team also looked separately at participants’ smoking history and exposure to LAN and observed that past and current smokers with higher exposures to light at night also had a higher incidence of breast cancer. James says they “don’t necessarily have an explanation for the mechanism through which there’s an interaction between smoking and light at night, because that was an unexpected finding,” but that “smoking has also been shown to decrease secretion of melatonin within this cohort.” The idea is that somehow, this is a “dual hit” where one melatonin-reducing factor compounds the other, resulting in an even greater elevated risk of breast cancer than either risk factor alone.

Although the team also looked for a clear correlation between shift work and higher incidence of breast cancer, James says they did not find a “statistically significant” increase among participants who work nights, even though by definition, their occupations would expose them to more light at night. But, even though the increase did not meet the criteria to be statistically significant in this particular study, James says they “did see that the association looked slightly stronger amongst participants who worked night shifts.”

Other studies have linked shift work with higher incidence of cancer, particularly breast and prostate cancer, both of which are often hormonally driven. James says this increase could come from a disruption of circadian patterns and melatonin secretion, but also possibly from a change in behavior patterns. Ever pull an all-nighter and wonder why you felt hungry all night? Blame your disrupted circadian rhythm.

James says the connection between shift work and increased risk of cancer is probably another example of a “dual hit. You’re getting your circadian patterns disrupted by working night shifts and you’re also living in an area with higher levels of light at night. It might be something where the combination of the two is sufficient to really disrupt your circadian patterns and lead to breast cancer risk.” He says this is an emerging field of research and one that requires a lot more work before we’ll fully understand what’s going on. But these findings are intriguing and suggest that circadian patterns may have a bigger impact on health outcomes than we realize.

Turn Out the Lights?!

For practical purposes, there’s no reason to panic just yet. “I would not elevate [LAN risk] to the level of other known risk factors for breast cancer,” James says. “Before I would suggest people move to the country to avoid light at night, I would probably first think about getting adequate levels of physical activity or perhaps eating a healthy diet that might be protective against breast cancer.”