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2025
Int J Environ Res Public Health
A prospective study of 10,305 women in Nova Scotia found that frequent sleep trouble (“all of the time”) was associated with a 141% increased breast cancer risk, while high physical activity was associated with a 42% reduced risk. Interestingly, sleep duration itself showed no association with breast cancer risk, suggesting that sleep quality—rather than quantity—may be the more important factor. These findings add sleep disturbances to the list of modifiable lifestyle risk factors for breast cancer and reinforce the protective benefits of physical activity, though the authors note that further research is needed to understand the biological mechanisms linking poor sleep quality to increased cancer risk.
2024
Ann Med
A study of 1,789 participants (including 263 breast cancer patients) from the National Health and Nutrition Examination Survey (NHANES) examined the associations between sleep duration, depression, and breast cancer risk, and developed machine learning algorithms to predict breast cancer. The study found that participants with depression had nearly double the odds of breast cancer (OR = 1.99; 95% CI: 1.55-3.51), while no significant association was observed between sleep duration (<7 hours, >9 hours compared to 7-9 hours) and breast cancer risk. Among six machine learning algorithms tested, the AdaBoost model performed best in predicting breast cancer with an area under the curve (AUC) of 0.84 (95% CI: 0.81-0.87), demonstrating good predictive capability. These findings suggest that depression, but not sleep duration, is significantly associated with increased breast cancer risk, providing new insights into the relationship between mental health and breast cancer while highlighting the need for further research into the underlying pathological mechanisms connecting depression and breast cancer development.
2022
Asian Pac J Cancer Prev
A matched case-control study of 187 breast cancer patients and 187 controls in Asia found that irregular sleep patterns and severe stress were the strongest modifiable risk factors, with irregular sleep associated with a staggering 3,311% increased breast cancer risk and severe stress showing a 574% increased risk. Poor sleep quality showed an even more dramatic 1,029% increased risk, while regular multivitamin use was associated with a 238% increased risk—a surprising finding requiring further investigation—and having a first child before age 30 was protective with a 56% risk reduction. Notably, none of the traditional non-modifiable risk factors (such as family history) showed significant associations in this study, suggesting that modifiable lifestyle factors—particularly sleep quality and stress management—may be critically important targets for breast cancer prevention in Asian populations experiencing rapidly rising breast cancer rates.
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.
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.