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2025
Environ Health Perspect
This nested case-control study, part of the E3N-Generations cohort of France, used satellite data to estimate exposures to both outdoor light-at-night. They found a linear increase in breast cancer risk, based upon quartiles or outdoor light-at-night exposure, p<.01. When they controlled for nitrogen dioxide exposure, pm 2.5, and vegetation density, the trend was no longer significant, but breast cancer risk was still elevated in association with light-at-night exposure (OR=1.11, 95%CI=1.02-1.20).
2023
Environ Res
A nationwide U.S. study of nearly 45,000 women found clear geographic patterns in breast cancer rates, with lower risk in the South and Southeast and higher risk in the Northwest and parts of the Midwest and Northeast, even after accounting for personal risk factors like family history and reproductive factors. Environmental exposures—including air pollution (nitrogen dioxide and fine particles), light at night, greenspace, and neighborhood socioeconomic disadvantage—explained 21% of the geographic variation in overall breast cancer and 63% of the variation specifically for estrogen receptor-positive (ER+) breast cancer. These findings provide strong evidence that where you live matters for breast cancer risk, with environmental factors playing a substantial role, particularly for ER+ breast cancer, the most common subtype, suggesting that reducing environmental exposures like air pollution and light at night could be effective prevention strategies at the community level.
2021
Sci Total Environ
This meta-analysis combined data from multiple studies to examine whether exposure to artificial light at night (LAN) — both outdoor and indoor — is associated with breast cancer risk. Women in the highest exposure categories had about a 12% greater risk of developing breast cancer compared with those in the lowest exposure categories (summary relative risk = 1.12, 95% CI: 1.06–1.18 for outdoor LAN; 1.13, 95% CI: 1.05–1.21 for indoor LAN). The association was somewhat stronger for estrogen-receptor–positive tumors. The authors concluded that LAN is a plausible, albeit modest, environmental risk factor for breast cancer.
2021
Environ Pollut
This meta-analysis synthesized data from 6 observational studies that used satellite-based measures of outdoor “light at night” (LAN) exposure to assess associations with breast cancer risk. Women in the highest-exposure group had were more likely to be diagnosed with breast cancer compared than those in the lowest-exposure group (OR = 1.11, 95% CI: 1.06–1.16). In subgroup analyses, the association remained significant for postmenopausal women (OR ≈ 1.07, 95% CI: 1.00–1.13).
2020
Cancer Epidemiol
A study of 186,981 postmenopausal women in the NIH-AARP Diet and Health Study found that those living in areas with the highest levels of outdoor light at night (LAN) had a 10% increased risk of developing breast cancer over 16 years of follow-up compared to women in the darkest areas. The association was stronger for estrogen receptor-positive breast cancer (12% increased risk) and appeared to vary by individual characteristics such as smoking, alcohol consumption, sleep duration, and body mass index. These findings suggest that nighttime light pollution may disrupt circadian rhythms and contribute to breast cancer development, particularly hormone-sensitive tumors. The results highlight artificial light at night as a potentially modifiable environmental risk factor for postmenopausal breast cancer, with implications for urban planning and outdoor lighting policies.
2018
Eur J Epidemiol
This large international study pooling data from over 13,000 women across five countries found that night shift work increased breast cancer risk by 26% in pre-menopausal women, with the risk rising substantially for those working longer shifts (≥10 hours), more frequent nights (≥3 nights per week), or longer durations (≥10 years). Pre-menopausal women working both long durations and high frequency had a 2.5 times higher breast cancer risk, with current or recent night workers at higher risk than those who had stopped more than two years ago. Notably, no increased risk was found in post-menopausal women, and the elevated risk was primarily for estrogen receptor-positive tumors, particularly those that were also HER2-positive, suggesting that disruption of hormones and circadian rhythms during reproductive years may be key factors in how night work affects breast cancer risk.
2024
Sci Tot Environ
A comprehensive systematic review and meta-analysis of 28 studies including over 2.5 million individuals found that higher levels of outdoor artificial light at night (ALAN) were associated with a 12% increased breast cancer risk, though indoor ALAN showed a non-significant 7% increased risk, with no differences by menopausal status. For prostate cancer, the analysis suggested a 43% increased risk with outdoor ALAN exposure, though this was not statistically significant, and qualitative synthesis revealed positive associations with non-Hodgkin lymphoma, colorectal, pancreatic, and thyroid cancers. The authors note a critical limitation: most studies relied on low-resolution satellite imagery (1-5 km resolution from the Defense Meteorological Program) without information on light color or spectral composition, which may have led to exposure misclassification and underestimation of true effects, highlighting the urgent need for studies using higher-resolution exposure assessment methods and investigation of light pollution effects on cancers beyond breast cancer.
2023
Frontiers
A meta-analysis of 21 studies including 734,372 participants worldwide found that light at night (LAN) exposure was associated with a 12% increased breast cancer risk overall, with case-control studies showing 16% increased risk and cohort studies showing 8% increased risk. The association was particularly pronounced in Asian populations (24% increased risk) and for ER-positive breast cancers (10% increased risk), while outdoor LAN specifically showed a 7% increased risk. These findings support the hypothesis that artificial light exposure at night disrupts circadian rhythms and suppresses melatonin production—a hormone with anti-cancer properties—though the authors caution against taking melatonin supplements for prevention without medical advice until mechanisms are better understood, and emphasize the need for high-quality research accounting for environmental confounding factors to clarify the role of light pollution in breast cancer development.
2023
Occup Environ Med
A prospective cohort study of 33,359 Finnish public sector workers followed until 2016 found that shift work—both with and without night shifts—was associated with approximately double the breast cancer risk among women aged 50 or older after 10 or more years of follow-up (101% and 105% increased risk, respectively). However, no overall association was found when examining the entire cohort regardless of age or duration, and when historical shift work exposure data were available in a subgroup, the association with longer exposure duration was not statistically significant. These findings suggest that the breast cancer risk from shift work may primarily affect older women with prolonged exposure, though the authors acknowledge that incomplete information on the intensity and patterns of night work exposure may have weakened the observed associations.
2021
Int J Health Geogr
A systematic review and dose-response meta-analysis of 17 studies found that exposure to light at night (LAN)—both outdoor and indoor—was associated with an 11% increased breast cancer risk overall, with slightly stronger associations in premenopausal women (16% increased risk) and ER-positive breast cancers (9% increased risk). The dose-response analysis of outdoor LAN showed a linear relationship up to 40 nW/cm²/sr, after which the curve flattened, particularly among premenopausal women, suggesting a threshold effect. These findings provide the first comprehensive dose-response assessment of the LAN-breast cancer relationship and support growing concerns about light pollution as an environmental breast cancer risk factor, with implications for urban planning, building design, and personal light exposure habits—particularly for younger women who appear most vulnerable to the carcinogenic effects of circadian disruption from artificial light exposure at night.
2021
Environ Res
A prospective cohort study of 16,941 Danish nurses followed from 1993/1999 through 2012 (745 breast cancer cases during 320,289 person-years) examined residential outdoor light at night (LAN) exposure using satellite data and found no overall association between LAN and breast cancer risk in fully adjusted models accounting for individual characteristics, traffic noise, and air pollution, with hazard ratios of 0.97 (95% CI: 0.77-1.23) for highest LAN exposure and 1.09 (95% CI: 0.90-1.31) for medium exposure compared to lowest tertile. A suggestive association was observed between LAN and estrogen receptor-negative (ER-) breast cancer, though this was not statistically significant. Despite mechanistic evidence from melatonin research and xenograft experiments suggesting that light at night exposure may disrupt circadian rhythms and increase breast cancer risk, this large prospective study found only weak evidence for such an association using objective outdoor LAN measurements, though the study was limited to outdoor residential exposure and may not capture indoor lighting exposure or occupational night work that could be more relevant to circadian disruption and breast cancer development.
2019
Chronobiol Int
A nested case-control study of 39,686 postmenopausal women in the California Teachers Study found that women with a definite evening chronotype (“night owls”) had a 20% increased breast cancer risk compared to definite morning chronotypes (“morning larks”), even after adjusting for established breast cancer risk factors. Importantly, this association was observed in a population without substantial night shift work history, suggesting that chronotype itself—the behavioral manifestation of an individual’s underlying circadian rhythm—may be an independent breast cancer risk factor beyond the effects of occupational circadian disruption. These findings raise the intriguing possibility that evening chronotypes may be more susceptible to environmental circadian disruption from factors like artificial light exposure, social jet lag (mismatch between biological and social timing), or irregular sleep-wake patterns, and warrant further investigation in other non-shift worker populations to confirm whether innate circadian preference represents a novel, modifiable risk factor for breast cancer through behavioral interventions targeting sleep timing and light exposure patterns.