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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).
2022
J Occup Environ Med
A systematic review and meta-analysis of 9 studies found that female flight attendants had a 43% increased breast cancer incidence compared to the general population, but surprisingly, neither cosmic radiation exposure nor circadian rhythm disruption—the two primary occupational hazards hypothesized to drive this excess risk—showed clear associations with breast cancer in the available studies. Three studies suggested a possible link with cosmic radiation while none found associations with circadian disruption, leaving the underlying cause of the elevated breast cancer risk among flight attendants unexplained. These findings highlight a critical gap in occupational health research: while flight attendants clearly face elevated breast cancer risk, the mechanisms remain unclear, potentially involving unmeasured factors such as reproductive patterns (delayed childbearing, fewer children), lifestyle factors associated with the profession, cumulative effects of multiple low-level exposures, or limitations in exposure assessment methods—underscoring the urgent need for studies with detailed individual-level data on occupational exposures, work schedules, and lifestyle factors to identify modifiable risk factors for this vulnerable workforce.
2020
Medicina
A systematic review of 12 studies examining nurses and shift work found that most studies showed an association between breast cancer and consecutive rotating night shifts prolonged over time, with risk increasing particularly during early adulthood and after 5 or more years of working 6 or more consecutive night shifts. The review identified disruption of circadian rhythm and alterations in peripheral clock genes and reproductive hormones as key mechanisms linking night shift work to breast cancer development, with potential roles for melatonin suppression and epigenetic changes including telomere alterations. These findings are particularly concerning given that nursing is a predominantly female profession requiring 24-hour staffing, suggesting the need for workplace policies that limit consecutive night shifts and total years of night work exposure, along with further research to establish definitive causal mechanisms and identify protective strategies for the millions of women working night shifts globally.
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.
2019
BMJ
A Mendelian randomization study using UK Biobank data (156,848 women including 7,784 breast cancer cases) and Breast Cancer Association Consortium data (122,977 cases, 105,974 controls) examined causal effects of sleep traits on breast cancer risk using genetic variants associated with chronotype, sleep duration, and insomnia symptoms. Two-sample MR analysis confirmed that morning preference reduced breast cancer risk by 12% per category increase (OR = 0.88; 95% CI: 0.82-0.93) and provided suggestive evidence that each additional hour of sleep duration increased risk by 19% (OR = 1.19; 95% CI: 1.02-1.39) for both ER+ and ER- breast cancer subtypes, with inconsistent evidence for insomnia symptoms. These findings—robust to sensitivity analyses accounting for horizontal pleiotropy—provide genetic evidence that being a “morning person” may protect against breast cancer while longer sleep duration may increase risk, suggesting that circadian rhythm patterns and sleep duration represent modifiable risk factors, though the counterintuitive finding regarding sleep duration requires further investigation given that adequate sleep is generally considered health-protective.
2018
Int J Cancer
A population-based case-control study in Spain (2008-2013) including 621 prostate cancer cases, 1,205 breast cancer cases, and 2,193 controls who never worked night shifts examined whether meal timing is associated with cancer risk while accounting for lifestyle factors and chronotype (morning vs. evening preference). Participants who waited two or more hours between supper and sleep had a 20% reduced risk of breast and prostate cancer combined (OR = 0.80; 95% CI: 0.67-0.96) compared to those sleeping immediately after eating, with individual reductions of 26% for prostate cancer and 16% for breast cancer; similarly, eating supper before 9 pm versus after 10 pm showed protective effects, with stronger associations among those adhering to cancer prevention recommendations (OR = 0.65) and morning chronotypes (OR = 0.66). These findings suggest that adhering to diurnal eating patterns—particularly maintaining a long interval between the last meal and sleep—is associated with lower breast and prostate cancer risk, independent of diet quality and other lifestyle factors. The study highlights the emerging importance of meal timing and circadian rhythm alignment in cancer prevention, indicating that when we eat may be as important as what we eat, and suggesting that late-night eating close to bedtime may disrupt metabolic and hormonal processes that influence cancer development.