It has been known for a couple of decades that exposure to light at night increases the risk of breast cancer. This is mainly due to light at night decreasing melatonin production, since melatonin acts as an antioxidant and helps to prevent breast and colon cancers. The International Agency for Research on Cancer (part of the World Health Organization) listed light at night and shift work as a probable carcinogen in 2007.

So while it has been known since the late 80's that shift work seems to increase the risk of breast cancer, it has only more recently been found that overall artificial light at night correlates in a lot of places with breast cancer rates.

Researchers have looked at the interaction between dim light at night from urban light pollution and breast cancer incidence in millions of women in over 180 countries. The findings are not all the same, but the estimates range from a low of a 7% increase in risk to other studies that showed about a 50% increase in risk. (Compare this with many recent headlines about alcohol increasing the risk of breast cancer... by only 5%.)  Why isn't every government, every doctor, every talking-head health guru on TV telling us to put up some blackout curtains and dial down the lights at night???

Digging into the research studies....

Light at Night and Connecticut Breast Cancer:
A study that came out in August 2018 looked into the link between artificial light at night and breast cancer rates in Connecticut.  The study took into factors such as socio-demographic and lifestyle influences (alcohol, obesity, exercise, age, family history, shift work). Researchers used geo-information tools to normalize aerial density of breast cancer incidences according to the population density and risk factors. They found a 'halo' geographic pattern of increased breast cancer incidences in areas around the main cities in CT.  This matched their model for artificial light at night. The researchers concluded:

Our models revealed that ALAN emerged as a statistically significant and independent predictor for BC rates in all models, after controlling for population density, fertility rate, percents of non-white population and population below poverty.

World-wide investigation of 180 countries:
A 2015 study looked at breast cancer rates in 180 countries and found that light at night was statistically linked to breast cancer risk.  The study controlled for country-level differences such as birth rates, percentage of urban population, and GDP. While there was an overall statistical link, the researchers did find that the level of association between light at night and breast cancer was highest in countries in Western Europe.  Countries in South East Asia and Gulf States didn't have nearly as strong of an increased risk from their relatively high light at night.  The researchers theorized that this could be due to local diet and lifestyle.

Spanish study:
A study of women in Barcelona and Madrid found that women who were exposed to more light at night were at a 47% increased risk of breast cancer (and men at a 2x risk for prostate cancer...)  This study excluded women who had ever worked the night shift. The researchers used personal interviews to estimate indoor light at night exposure due to chronotype (staying up late), eReaders or other screen exposure, and average bedtime and wake time.  The researcher then used imaging from the International Space Station to determine the ground level spectral type of lamps and light emissions. They also were able to calculate the amount of blue light at a given area on a per pixel basis from the ISS photos, which were mapped to the house locations of each participant.

Study of 100,000+ US women:
Another study in 2017 used the health data from 100,000 US women (nurses) who had been followed for over 20 years.  After adjusting for all the usual lifestyle, socioeconomic, and anthropometric factors, the researchers still found that light at night increased the risk of breast cancer in premenopausal women. For premenopausal women the increased risk was 7% and for women who were former or current smokers that risk was 21% higher.

California teacher study:
A study of 106,000 teachers in California found that light at night increased the risk of breast cancer in premenopausal women by 34%. The researchers based this on answers to a 1996 questionnaire on lighting at night compiled together with satellite images at night from 1996/97.  While probably not as accurate as some of the other methods of determining light at night, it is interesting to see how they can use data from a 1996 survey compared with a 20+ year followup on cancer incidences in the group.

British study showing no link between cancer and light at night:
A study in the British Journal of Cancer in 2018 made headlines for its results showing no statistical link between light at night and cancer. I encourage everyone to read the study for themselves... Basically, the researchers mailed out a questionnaire asking 100,000 women to report how much light they were exposed to at night at age 20.  Seriously. This peer-reviewed report made huge headlines by showing no link between light at night and breast cancer -- based on a questionnaire asking women to remember how much light there was in their bedrooms while they slept when they were 20! At least 75% of the participants were born before 1970. So the researchers were asking these women to think back at least 30 years and remember how much light was in their bedrooms at night.  I can't remember what I ate for dinner a couple of days ago, much less how much light was around when I was 20.

Animal studies and cell studies:
While the huge studies looking at populations and light via satellites are cool, they don't tell us why there is a link between light and breast cancer.  There have been quite a few studies done on animals, though, to try to determine how / why light at night increases the risk of breast cancer. Animals studies and cell studies allow scientists to manipulate the variables and isolate the reasons 'why'.

A study in mice found that growth rates for tumors were greatest when the mice were exposed to light at night. This increase in tumor growth could be reversed, though, with melatonin. This points to melatonin preventing breast cancer, which is something that has been shown in cell studies and in human studies as an adjunct to chemotherapy.

A 2005 study of human breast cancer cells found that cancer growth was suppressed when they were perfused with melatonin rich blood taken from premenopausal women during the night.  Those same cancer cells, though, had higher tumor proliferation when exposed to blood from the same premenopausal women after they had been exposed (at night) to white fluorescent light for 90 minutes.

A mouse study found that dim light at night not only 'speeds the development of breast tumors", but it also confers 'an intrinsic resistance to tamoxifen therapy.' They were able to reverse the effects of dim light at night by giving the mice supplemental melatonin.