Domestic light at night and breast cancer risk: a prospective analysis of 105 000 UK women in the Generations Study

Exposure to Light at Night and Risk of Cancer: A Systematic Review, Meta-Analysis, and Data Synthesis

BACKGROUND

Emerging interest surrounds the role of environmental factors, notably exposure to light at night (LAN), as a potential cause of cancer. The aim of this study was to conduct a systematic review and, if possible, meta-analysis of observational studies on LAN and cancer risk of multiple types.

METHODS

A systematic literature search in PubMed, Web of Science, and Embase, spanning from inception to May 2023, was conducted. Studies focusing on the association between LAN exposure and cancer risk in adult populations were included. We used random effects models to calculate pooled risk estimates (RR) and 95% confidence intervals (CI). We assessed study quality using the Risk of Bias in Non-randomized Studies of Interventions.

RESULTS

Among 8492 initially identified studies, 26 met the inclusion criteria (13 were case-control and 13 were cohort studies). These studies were published from 2001 to 2023 and assessed diverse cancer types in North America, Asia, Europe, and Australia. Except for breast cancer, there was a paucity of site-specific cancer studies. In the meta-analysis of 19 breast cancer studies, higher exposure to indoor (summary RR, 1.08; 95% CI 1.01-1.15) and outdoor (summary RR, 1.10; 95% CI, 1.04-1.15) LAN were associated with increased risk. After excluding one low-quality study, the results were unchanged.

CONCLUSIONS

We found a positive association between LAN exposure and breast cancer risk in women. However, data are lacking for other cancer types, and further studies are required to better understand the role of LAN on cancer.

Digital Screen Time and the Risk of Female Breast Cancer: A Retrospective Matched Case-Control Study

Background

As the use of electronic devices such as mobile phones, tablets, and computers continues to rise globally, concerns have been raised about their potential impact on human health. Exposure to high energy visible (HEV) blue light, emitted from digital screens, particularly the so-called artificial light at night (ALAN), has been associated with adverse health effects, ranging from disruption of circadian rhythms to cancer. Breast cancer incidence rates are also increasing worldwide.

Objective

This study aimed at finding a correlation between breast cancer and exposure to blue light from mobile phone.

Material and Methods

In this retrospective matched case-control study, we aimed to investigate whether exposure to blue light from mobile phone screens is associated with an increased risk of female breast cancer. We interviewed 301 breast cancer patients (cases) and 294 controls using a standard questionnaire and performed multivariate analysis, chi-square, and Fisher's exact tests for data analysis.

Results

Although heavy users in the case group of our study had a statistically significant higher mean 10-year cumulative exposure to digital screens compared to the control group (7089±14985 vs 4052±12515 hours, respectively, P=0.038), our study did not find a strong relationship between exposure to HEV and development of breast cancer.

Conclusion

Our findings suggest that heavy exposure to HEV blue light emitted from mobile phone screens at night might constitute a risk factor for promoting the development of breast cancer, but further large-scale cohort studies are warranted.

Considering the impact of shift working on health

Shift work is necessary for the provision of health services, especially in hospitals as it is the only way to provide continuous care to patients. Working at night is inconsistent with the body's biological clock and night shift working hugely disrupts the circadian and sleep wake cycles. In 2020, the World Health Organization classified night shift working as being a 'probable' class 2A carcinogen and a number of independent cohort studies of both night workers and shift workers have observed increased incidence of certain cancers. Altered sleep timing, food timing and light exposure typical for shift workers leads to an acute circadian disruption and misalignment of regulatory hormones. This straying from synchronisation has been shown to result in individuals being more vulnerable to a number of chronic health conditions. Night shift work disrupts the normal sleep-wake cycle, often leading to shortened sleep duration and excessive fatigue and the results of this are far reaching. This article will explore the full impact of shift working, questioning if current practices employed within the NHS should continue.

Breast cancer: Epidemiology, risk factors and screening

Breast cancer is a global health concern with a significant impact on the well-being of women. Worldwide, the past several decades have witnessed changes in the incidence and mortality of breast cancer. Additionally, epidemiological data reveal distinct geographic and demographic disparities globally. A range of modifiable and non-modifiable risk factors are established as being associated with an increased risk of developing breast cancer. This review discusses genetic, hormonal, behavioral, environmental, and breast-related risk factors. Screening plays a critical role in the effective management of breast cancer. Various screening modalities, including mammography, ultrasound, magnetic resonance imaging (MRI), and physical examination, have different applications, and a combination of these modalities is applied in practice. Current screening recommendations are based on factors including age and risk, with a significant emphasis on minimizing potential harms to achieve an optimal benefits-to-harms ratio. This review provides a comprehensive insight into the epidemiology, risk factors, and screening of breast cancer. Understanding these elements is crucial for improving breast cancer management and reducing its burden on affected individuals and healthcare systems.

Light at night exposure and risk of breast cancer: a meta-analysis of observational studies

Objective

The aim of this meta-analysis is to evaluate the impact of light at night (LAN) exposure on the risk of breast cancer across varying factors.

Method

We conducted a systematic search of literature up to July 15, 2023, including PubMed, Cochrane Library, and Embase databases, using keywords related to breast cancer and LAN exposure. Cohort study and case-control study literature on night light exposure and breast cancer risk were included. Statistical analyses were performed using Stata software version 17.0. To address heterogeneity among different studies, we employed a random-effects model for analysis and assessed publication bias using funnel plots and Egger's test.

Results

We included 13 case-control and 8 cohort studies with 734,372 participants worldwide. In the Newcastle-Ottawa Scale (NOS) assessments, the average score was 7.43 (ranging from 5 to 9). The overall meta-analysis demonstrated a significant association between exposure to LAN and risk of breast cancer (RR = 1.12; 95% CI: 1.06-1.17; I2 = 31.3%, p < 0.001). In the subgroup analysis, the results of the analysis for study types (case-control studies: RR = 1.16; 95% CI: 1.06-1.27; I2 = 40.4%, p = 0.001; cohort studies: RR = 1.08; 95% CI: 1.04-1.14; I2 = 0.0%, p < 0.001) and the results for light exposure types (outdoor LAN: RR = 1.07; 95% CI: 1.02-1.13; I2 = 30.9%, p = 0.004) are presented. In the analysis conducted for continents, the highest breast cancer risk was observed in the Asian population (Asian: RR = 1.24; 95% CI: 1.15-1.34; I2 = 0.0%, p < 0.001) and in the analysis of estrogen receptor status (ER+: RR = 1.10; 95% CI: 1.03-1.18; I2 = 17.0%, p = 0.005;). We also conducted an analysis on menopausal status and various lifestyles but did not find any statistically significant findings.

Conclusion

Our study demonstrates that LAN exposure is associated with an increased risk of breast cancer, particularly in the Asian population. Among the existing hypotheses, the idea that LAN exposure leads to a decrease in melatonin is widely accepted. However, until the mechanism of this effect is clearly elucidated, it is not recommended to take melatonin supplements for breast cancer prevention without medical advice. We hope to conduct more high-quality research, especially concerning the investigation of other environmental confounding factors, to further advance this field.

Understanding light pollution: Recent advances on its health threats and regulations

The prevalence of artificial lights not only improves the lighting conditions for modern society, but also poses kinds of health threats to human health. Although there are regulations and standards concerning light pollution, few of them are based on the potential contribution of improper lighting to diseases. Therefore, a better understanding of the health threats induced by light pollution may promote risk assessment and better regulation of artificial lights, thereby a healthy lighting environment. This review is based on a careful collection of the latest papers from 2018 to 2022 about the health threats of light pollution, both epidemiologically and experimentally. In addition to summing up the novel associations of light pollution with obesity, mental disorders, cancer, etc., we highlight the toxicological mechanism of light pollution via circadian disruption, since light pollution directly interferes with the natural light-dark cycles, and damages the circadian photoentrainment of organisms. And by reviewing the alternations of clock genes and disturbance of melatonin homeostasis induced by artificial lights, we aim to excavate the profound impacts of light pollution based on accumulating studies, thus providing perspectives for future research and guiding relevant regulations and standards.

Outdoor artificial light at night and human health: A review of epidemiological studies

We conducted a non-systematic review of epidemiological studies on a potential link between exposure to outdoor artificial light at night (O-ALAN) and disease occurrence in humans published since 2009. In recent years, a number of presses have been published on this issue, but the conclusions have been mixed. We therefore decided to critically analyze the available epidemiological evidence of such a correlation. After a careful search, 51 studies were identified and included in the review. They addressed the potential link between O-ALAN exposure and the incidence of breast cancer, other cancers, sleep and circadian rhythm disorders, obesity and cardiovascular diseases, mental disorders, infectious diseases, and complications during pregnancy and childbirth. The vast majority of papers revealed the existence of such a link. However, the amount of epidemiological evidence supporting the correlation across groups of disorders varied widely. In addition, we found that all papers contained at least one of the following omissions: lack of the temporal and spatial resolution in light at night measurements, measuring only light intensity without considering its wavelength, and not accounting for many important confounding factors in their statistical analyses. Therefore, we believe that the link between O-ALAN exposure and the occurrence of the disorders in question suggested by the authors of the reviewed papers may be in some cases at least to some extent, a coincidence. Further epidemiological studies, free of significant omissions highlighted in this paper, are needed.

Circadian Disruption and Breast Cancer Risk: Evidence from a Case-Control Study in China

Studies had suggested an association between circadian disruptors (including night shift work, domestic light exposure at night, sleep duration, and circadian gene polymorphism) and breast cancer, while rare studies had been conducted in the Chinese population. This study was a case-control study conducted to explore the impact of circadian disruptors on the risk of breast cancer in China. Four hundred and sixty-four cases and 464 controls, admitted from the Department of Breast Surgery, Cancer Hospital, Chinese Academy of Medical Sciences, were included in this study. Adjusting age, BMI group, smoking, alcohol consumption, menopausal status, family history of breast cancer, duration of breastfeeding, age at menarche, number of pregnancies, age at first full-term pregnancy, use of estrogen and use of oral contraceptive, multivariate logistic regression analysis showed that the risk of breast cancer was higher in short sleep duration group (OR = 4.86, 95%CI: 1.73-17.33). Meanwhile, rs2292912 in CRY2, rs2253820 in PER1, rs2289591 in PER1 and rs3027188 in PER1 were positively associated with the risk of breast cancer. This study supported that the short duration of sleep and four SNPs in crucial circadian genes played a role in the development of breast cancer.

Light at night and the risk of breast cancer: Findings from the Sister study

BACKGROUND

Light at night (LAN) may alter estrogen regulation through circadian disruption. High levels of outdoor LAN may increase breast cancer risk, but studies have largely not considered possible residual confounding from correlated environmental exposures. We evaluated the association between indoor and outdoor LAN and incident breast cancer.

METHODS

In 47,145 participants in the prospective Sister Study cohort living in the contiguous U.S., exposure to outdoor LAN was determined using satellite-measured residential data and indoor LAN was self-reported (light/TV on, light from outside the room, nightlight, no light). We used Cox proportional hazards models to calculate hazard ratios (HRs) and 95% confidence intervals (CIs) for the associations between outdoor and indoor LAN and breast cancer risk. Models were adjusted for age, race/ethnicity, educational attainment, annual household income, neighborhood disadvantage, latitude, and population density as a proxy for urbanicity. To evaluate the potential for residual confounding of the outdoor LAN and breast cancer relationship by factors associated with urbanicity, we considered further adjustment for exposures correlated with outdoor LAN including NO2 [Spearman correlation coefficient, rho (ρ) = 0.78], PM2.5 (ρ = 0.36), green space (ρ =  - 0.41), and noise (ρ = 0.81).

RESULTS

During 11 years of follow-up, 3,734 breast cancer cases were identified. Outdoor LAN was modestly, but non-monotonically, associated with a higher risk of breast cancer (Quintile 4 vs 1: HR = 1.10, 95% CI: 0.99-1.22; Quintile 5 vs 1: HR = 1.04, 95% CI: 0.93-1.16); however, no association was evident after adjustment for correlated ambient exposures (Quintile 4 vs 1: HR = 0.99, 95% CI: 0.86-1.14; Quintile 5 vs 1: HR = 0.89, 95% CI: 0.74-1.06). Compared to those with no indoor LAN exposure, sleeping with a light or TV on was associated with a HR = 1.09 (95% CI: 0.97-1.23) in the adjusted model.

CONCLUSIONS

Outdoor LAN does not appear to increase the risk of breast cancer after adjustment for correlated environmental exposures.

Association between bedroom light exposure at night and allostatic load among Chinese young adults

Light at night (LAN) has received increasing attention for its potential health hazards to human and animals. However, to our knowledge, no study has explored the specific effects of bedroom nighttime light exposure on allostatic load (AL). To investigate the association between bedroom individual-level LAN exposure and AL among young adults, an integrative index manifests multiple system dysregulation. Using data from a cohort of 484 Chinese young adults aged 16-22 years. Bedroom light was objectively recorded at 1-min intervals for two nights using a portable illuminance meter. Fasting blood samples were collected at one-year follow-up for the detection of AL parameters. AL score was derived as sum of the top quartile of twelve physiological biomarkers in four systems: metabolic system (BMI, WC, TC, HDL, LDL, TG, HbA1c, INS, GLU); cardiovascular system (SBP, DBP); immune and inflammatory systems (hs-CRP), with HDL was lowest quartile. Univariate and multivariate linear regression models were used to evaluate the association between LAN intensity with AL score and separate AL parameters. The average age of subjects was 18.7 years, 64.3% were female. The mean AL score of LAN group (average LAN intensity ≥ 3lx) was significantly higher than Dim group (3.6 ± 2.6 vs. 2.7 ± 2.1; P = 0.007). For each 1 lx increase of LAN intensity was associated with 0.15-unit increase in AL score (95% CI: 0.06, 0.24; P = 0.001). Moreover, LAN group was associated with increased 1.01-unit in AL score (95% CI: 0.36-1.66; P = 0.003) compared to Dim group. Significant associations between bedroom LAN exposure with allostatic load and separate AL biomarkers were observed in our study. Keeping bedroom darkness at night may be a practicable option to reduce the wear of multiple body systems and improve human cardiometabolic health from early in life.

Triple-Negative Breast Cancer: A Brief Review About Epidemiology, Risk Factors, Signaling Pathways, Treatment and Role of Artificial Intelligence

Triple-negative breast cancer (TNBC) is a kind of breast cancer that lacks estrogen, progesterone, and human epidermal growth factor receptor 2. This cancer is responsible for more than 15-20% of all breast cancers and is of particular research interest as it is therapeutically challenging mainly because of its low response to therapeutics and highly invasive nature. The non-availability of specific treatment options for TNBC is usually managed by conventional therapy, which often leads to relapse. The focus of this review is to provide up-to-date information related to TNBC epidemiology, risk factors, metastasis, different signaling pathways, and the pathways that can be blocked, immune suppressive cells of the TNBC microenvironment, current and investigation therapies, prognosis, and the role of artificial intelligence in TNBC diagnosis. The data presented in this paper may be helpful for researchers working in the field to obtain general and particular information to advance the understanding of TNBC and provide suitable disease management in the future.

Verification, analytical validation and clinical validation (V3) of wearable dosimeters and light loggers

Background

Light exposure is an important driver and modulator of human physiology, behavior and overall health, including the biological clock, sleep-wake cycles, mood and alertness. Light can also be used as a directed intervention, e.g., in the form of light therapy in seasonal affective disorder (SAD), jetlag prevention and treatment, or to treat circadian disorders. Recently, a system of quantities and units related to the physiological effects of light was standardized by the International Commission on Illumination (CIE S 026/E:2018). At the same time, biometric monitoring technologies (BioMeTs) to capture personalized light exposure were developed. However, because there are currently no standard approaches to evaluate the digital dosimeters, the need to provide a firm framework for the characterization, calibration, and reporting for these digital sensors is urgent.

Objective

This article provides such a framework by applying the principles of verification, analytic validation and clinical validation (V3) as a state-of-the-art approach for tools and standards in digital medicine to light dosimetry.

Results

This article describes opportunities for the use of digital dosimeters for basic research, for monitoring light exposure, and for measuring adherence in both clinical and non-clinical populations to light-based interventions in clinical trials.

Light at night and risk of breast cancer: a systematic review and dose-response meta-analysis

Breast cancer is the most common malignancy in women and the second leading cause of cancer death overall. Besides genetic, reproductive, and hormonal factors involved in disease onset and progression, greater attention has focused recently on the etiologic role of environmental factors, including exposure to artificial lighting such as light-at-night (LAN). We investigated the extent to which LAN, including outdoor and indoor exposure, affects breast cancer risk. We performed a systematic review of epidemiological evidence on the association between LAN exposure and breast cancer risk, using a dose–response meta-analysis to examine the shape of the relation. We retrieved 17 eligible studies through September 13, 2021, including ten cohort and seven case–control studies. In the analysis comparing highest versus lowest LAN exposure, we found a positive association between exposure and disease risk (risk ratio [RR] 1.11, 95% confidence interval-CI 1.07–1.15), with comparable associations in case–control studies (RR 1.14, 95% CI 0.98–1.34) and cohort studies (RR 1.10, 95% CI 1.06–1.15). In stratified analyses, risk was similar for outdoor and indoor LAN exposure, while slightly stronger risks were observed for premenopausal women (premenopausal: RR 1.16, 95% CI 1.04–1.28; postmenopausal: 1.07, 95% CI 1.02–1.13) and for women with estrogen receptor (ER) positive breast cancer (ER + : RR 1.09, 95% CI 1.02–1.17; ER–: RR 1.07, 95% CI 0.92–1.23). The dose–response meta-analysis, performed only in studies investigating outdoor LAN using comparable exposure assessment, showed a linear relation up to 40 nW/cm²/sr after which the curve flattened, especially among premenopausal women. This first assessment of the dose–response relation between LAN and breast cancer supports a positive association in selected subgroups, particularly in premenopausal women.

Mummy Induces Apoptosis Through Inhibiting of Epithelial-Mesenchymal Transition (EMT) in Human Breast Cancer Cells

Background:

Mummy (Iranian pure shilajit) is a remedy with possessing anti-inflammatory, antioxidant and anticancer activities. This study aimed to examine mummy effects on epithelial-mesenchymal transition (EMT) and invasiveness of MCF-7 and MDA-MB-231 breast cancer (BC) cell lines with underlying its mechanism.

Materials and Methods:

The dose-dependent inhibitory effect of the mummy on cell proliferation in vitro was determined using the MTT assay. Flow cytometry and 4’,6-diamidino-2-phenylindole dihydrochloride staining were respectively used for quantitative and qualitative analysis of cellular apoptosis, and gene expression analysis was conducted using real-time PCR.

Results:

MDA-MB-231 showed more sensitivity than the MCF-7 cell line to the anticancer activity of mummy, while mummy did not exhibit significant cell cytotoxicity against human normal cells (MCF-10A). The gene expression profile demonstrated a significant decrease in TGF-β1, TGF-βR1, TWIST1, NOTCH1, CTNNB1, SRC along with an increase in E-cadherin mRNA levels in mummy treated cells compared to the untreated control group (P≤0.05).

Conclusion:

Mummy triggers inhibition of EMT and metastasis in breast cancer cells mainly through the downregulation of TGFβ1 activity, and more studies required to find its specific anticancer activity with details.

Breast Cancer-Epidemiology, Risk Factors, Classification, Prognostic Markers, and Current Treatment Strategies-An Updated Review

Simple Summary

Breast cancer is the most common cancer among women. It is estimated that 2.3 million new cases of BC are diagnosed globally each year. Based on mRNA gene expression levels, BC can be divided into molecular subtypes that provide insights into new treatment strategies and patient stratifications that impact the management of BC patients. This review addresses the overview on the BC epidemiology, risk factors, classification with an emphasis on molecular types, prognostic biomarkers, as well as possible treatment modalities.

Abstract

Breast cancer (BC) is the most frequently diagnosed cancer in women worldwide with more than 2 million new cases in 2020. Its incidence and death rates have increased over the last three decades due to the change in risk factor profiles, better cancer registration, and cancer detection. The number of risk factors of BC is significant and includes both the modifiable factors and non-modifiable factors. Currently, about 80% of patients with BC are individuals aged >50. Survival depends on both stage and molecular subtype. Invasive BCs comprise wide spectrum tumors that show a variation concerning their clinical presentation, behavior, and morphology. Based on mRNA gene expression levels, BC can be divided into molecular subtypes (Luminal A, Luminal B, HER2-enriched, and basal-like). The molecular subtypes provide insights into new treatment strategies and patient stratifications that impact the management of BC patients. The eighth edition of TNM classification outlines a new staging system for BC that, in addition to anatomical features, acknowledges biological factors. Treatment of breast cancer is complex and involves a combination of different modalities including surgery, radiotherapy, chemotherapy, hormonal therapy, or biological therapies delivered in diverse sequences.

Associations of long-term exposure to environmental noise and outdoor light at night with age at natural menopause in a US women cohort

Previous studies have suggested noise, especially at night time, and light at night (LAN) could cause neuroendocrine disturbance and circadian disruption, which may lead to ovarian follicle atresia and earlier onset of menopause. However, no study to date has directly investigated the associations of exposure to these factors and menopausal age.

Methods

Premenopausal women from the Nurses' Health Study II (NHS II) were followed from age 40 through 2015. Median daytime and nighttime anthropogenic noise and outdoor LAN exposure were measured from a geospatial prediction model and satellite images, respectively, at residential addresses throughout the follow-up. Time-varying Cox proportional hazard models were used to calculate the hazard ratios and 95% confidence intervals, adjusting for individual lifestyle, reproductive history, and neighborhood socioeconomic factors. Possible effect modification by region, smoking status, body mass index, race/ethnicity, history of rotating shift work, and census tract population density and median income was examined.

Results

A total of 63,380 of 105,326 women self-reported natural menopause during 1,043,298 person-years of follow-up. No associations were found for noise (both daytime and nighttime) and outdoor LAN exposure with age at natural menopause (hazard ratios = 0.99-1.00) in the fully adjusted models. Sensitivity analyses showed similar null associations. No meaningful effect modification was found for region, smoking status, body mass index, race/ethnicity, history of rotating shift work, and census tract socioeconomic measures in stratified analyses.

Conclusion

No associations were found between environmental noise and outdoor LAN exposure in mid-adulthood and menopausal age in this cohort of US women.

Exposure to light at night (LAN) and risk of breast cancer: A systematic review and meta-analysis

BACKGROUND

With the unprecedented urbanization light pollution has emerged as a ubiquitous problem, and there has been accumulating evidence on the links between exposure to light at night (LAN) and breast cancer risk. We conducted a systematic review and meta-analysis of published studies on the associations between LAN exposure and breast cancer risk.

METHODS

We included all observational human studies wherein the exposure variable was LAN measured in indoor and outdoor environments, and the outcome was breast cancer. We employed summary relative risks (SRR) for breast cancer by comparing highest versus lowest categories of LAN exposure within a random-effects model. The National Toxicology Program's (NTP) Office of Health Assessment and Translation (OHAT) risk of bias rating tool was adopted to assess the risk of bias in individual studies and the Grading of Recommendations Assessment, Development and Evaluation (GRADE) guideline was employed to assess confidence in the body of evidence.

RESULTS

A total 14 studies comprising four cohorts (13,155 cases among 372,802 exposed subjects), nine case-control and one case-referent studies of female subjects (39,462 cases and 20,739 controls) across seven countries and published between 2001 and 20 were included for review. Participants in the highest LAN exposure category were associated with higher risk of breast cancer in reference to those in the lowest (SRR: 1.12; 95% CI: 1.06-1.18; I² = 39% for outdoor LAN, and SRR: 1.13; 95%CI: 1.05-1.21; I² = 19% for indoor LAN). Pooled evidence identified relatively pronounced association of outdoor LAN exposure and breast cancer among women with estrogen receptor positive (ER+) tumor (SRR: 1.21; 95% CI: 1.04-1.40) and premenopausal status (SRR: 1.21; 95% CI: 1.06-1.37). The final rate of confidence in the body of evidence generated was graded as 'moderate' based on GRADE guideline.

DISCUSSION

LAN exposure was consistently associated with higher breast cancer risk corroborating NTP's recommendations which anticipates excessive LAN as human carcinogen.

Clock at the Core of Cancer Development

To synchronize various biological processes with the day and night cycle, most organisms have developed circadian clocks. This evolutionarily conserved system is important in the temporal regulation of behavior, physiology and metabolism. Multiple pathological changes associated with circadian disruption support the importance of the clocks in mammals. Emerging links have revealed interplay between circadian clocks and signaling networks in cancer. Understanding the cross-talk between the circadian clock and tumorigenesis is imperative for its prevention, management and development of effective treatment options. In this review, we summarize the role of the circadian clock in regulation of one important metabolic pathway, insulin/IGF1/PI3K/mTOR signaling, and how dysregulation of this metabolic pathway could lead to uncontrolled cancer cell proliferation and growth. Targeting the circadian clock and rhythms either with recently discovered pharmaceutical agents or through environmental cues is a new direction in cancer chronotherapy. Combining the circadian approach with traditional methods, such as radiation, chemotherapy or the recently developed, immunotherapy, may improve tumor response, while simultaneously minimizing the adverse effects commonly associated with cancer therapies.

Sleep duration and breast cancer incidence: results from the Million Women Study and meta-analysis of published prospective studies

STUDY OBJECTIVES

To investigate the association between sleep duration and breast cancer incidence, we examined the association in a large UK prospective study and conducted a meta-analysis of prospective studies.

METHODS

In the Million Women Study, usual sleep duration over a 24-h period was collected in 2001 for 713,150 participants without prior cancer, heart problems, stroke, or diabetes (mean age = 60 years). Follow-up for breast cancer was by record linkage to national cancer registry data for 14.3 years on average from the 3-year resurvey. Cox regression models yielded multivariable-adjusted breast cancer relative risks (RR) and 95% confidence intervals (CIs) for sleep duration categories. Published prospective studies of sleep duration and breast cancer risk were included in a meta-analysis, which estimated the inverse-variance weighted average of study-specific log RRs for short and for long versus average duration sleep.

RESULTS

After excluding the first 5 years to minimize reverse causation bias in the Million Women Study, 24,476 women developed breast cancer. Compared with 7-8 h of sleep, the RRs for <6, 6, 9, and >9 h of sleep were 1.01 (95% CI, 0.95-1.07), 0.99 (0.96-1.03), 1.01 (0.96-1.06), and 1.03 (0.95-1.12), respectively. In a meta-analysis of 14 prospective studies plus the Million Women Study, including 65,410 breast cancer cases, neither short (RR < 7 h = 0.99 [0.98-1.01]) nor long (RR > 8 h = 1.01 [0.98-1.04]) versus average duration sleep was associated with breast cancer risk.

CONCLUSIONS

The totality of the prospective evidence does not support an association between sleep duration and breast cancer risk.

Exposure to artificial light at night and risk of cancer: where do we go from here?

Despite experimental and mechanistic data suggesting circadian disruption’s role in carcinogenesis, mixed findings from epidemiological investigations of artificial light at night and cancer risk in the general population are difficult to interpret due to exposure assessment limitations. It will be important for future studies to assess and validate individual-level exposures, ideally over the lifetime.

Exposure to outdoor light at night and risk of breast cancer: A systematic review and meta-analysis of observational studies

Recent epidemiological studies have explored effects of light at night (LAN) exposure on breast cancer, but reported inconsistent findings. We performed a systematic review and meta-analysis of available evidence regarding the association of LAN assessed by satellite data with breast cancer. We conducted a systematic PubMed, Web of Science, and EMBASE database literature search until August 2020. Random-effects meta-analysis was applied to synthesis risk estimates. Heterogeneity was measured using statistics of Cochran's Q, I², and Tau² (τ²). We assessed publication bias through funnel plot and Egger's test. Moreover, subgroup analyses according to study design and menopausal status were performed. Risk of bias (RoB) of each included study was assessed using a domain-based RoB assessment tool. The confidence in the body of evidence was appraised using the GRADE approach for level-of-evidence translation. A total of 1157 studies were identified referring to LAN and breast cancer, from which 6 were included for quantitative synthesis. We found a significantly higher odds of breast cancer in the highest versus the lowest category of LAN exposure (OR = 1.11, 95% CI: 1.06, 1.16; I² = 0.0%). In the subgroup analyses stratified by menopausal status and study design, significant association was found in postmenopausal women (OR = 1.07, 95% CI = 1.00, 1.13) and cohort studies (OR = 1.11, 95% CI = 1.05, 1.18), while the summary estimates of premenopausal women and case-control studies showed no significance. The level of evidence for the association of LAN exposure and breast cancer risk was graded as "moderate" with "probably low" RoB according to the NTP/OHAT framework. In conclusion, this study suggests a link of LAN exposure with risk of breast cancer. Further high-quality prospective studies, especially performed in low-to middle-income countries with improvement in the area of LAN exposure assessment are needed to advance this field.

Shift Work and Breast Cancer

The rates of shift work outside of daylight hours have increased in recent years, and nighttime shift work is now considered a potential carcinogenic occupational exposure. Light at night exposure, lower melatonin production, and the production of stress-related mediators disrupt normal sleep–wake cycles. Women who work lower-wage jobs and part-time workers whose shifts are determined entirely by their supervisors (rotating shifts) may be subject to stress related to efforts to align childcare and other needs with the unpredictable nature of rotating shift work. The causal link between breast cancer and the sleep cycle or circadian disruption are yet to be established; however, disruption of the circadian cycles by light at night exposure or chronic exposure to stress-related mediators have all been linked to the increased risk of breast cancer. We review the existing literature on shift work and breast cancer, identify knowledge gaps, and suggest future directions for research.

Light Pollution and Cancer

For many individuals in industrialized nations, the widespread adoption of electric lighting has dramatically affected the circadian organization of physiology and behavior. Although initially assumed to be innocuous, exposure to artificial light at night (ALAN) is associated with several disorders, including increased incidence of cancer, metabolic disorders, and mood disorders. Within this review, we present a brief overview of the molecular circadian clock system and the importance of maintaining fidelity to bright days and dark nights. We describe the interrelation between core clock genes and the cell cycle, as well as the contribution of clock genes to oncogenesis. Next, we review the clinical implications of disrupted circadian rhythms on cancer, followed by a section on the foundational science literature on the effects of light at night and cancer. Finally, we provide some strategies for mitigation of disrupted circadian rhythms to improve health.

Outdoor light at night and postmenopausal breast cancer risk in the NIH-AARP diet and health study

Circadian disruption may play a role in breast carcinogenesis. Previous studies reported relationships between outdoor light at night (LAN) and the breast cancer risk, but their findings are mixed. There is also a need to examine LAN and breast cancer incidence according to different individual and environmental characteristics to identify subpopulations at greater risk associated with LAN exposure. We studied residential outdoor LAN estimated from satellite imagery at baseline (1996) in relation to postmenopausal breast cancer incidence over ~16 years of follow-up in 186 981 postmenopausal women including 12 318 incident postmenopausal breast cancer cases in the NIH-AARP Diet and Health Study. We used Cox proportional hazards models to estimate hazard ratios (HR) and two-sided 95% confidence intervals (CI) of the relationship between quintiles of LAN and postmenopausal breast cancer risk, overall and by hormone receptor status and cancer stage. We found that when compared to women in the lowest quintile of baseline LAN, those in the highest quintile had a 10% increase in postmenopausal breast cancer risk (HR (95% CI), 1.10 (1.02, 1.18), P-trend, .002). The association appeared to be stronger for estrogen receptor (ER) positive breast cancer (1.12 [1.02, 1.24], .007) than for ER-negative cancer (1.07 [0.85, 1.34], .66). Our findings also suggested that the relationship between LAN and breast cancer risk may differ by individual characteristics, such as smoking, alcohol drinking, sleep duration and BMI, and neighborhood environment. In conclusion, our study suggests that higher outdoor LAN exposure may be a risk factor for postmenopausal breast cancer.

Circadian disruption: What do we actually mean?

The circadian system regulates physiology and behavior. Acute challenges to the system, such as those experienced when traveling across time zones, will eventually result in re-synchronization to local environmental time cues, but this re-synchronization is oftentimes accompanied by adverse short-term consequences. When such challenges are experienced chronically, adaptation may not be achieved, as for example in the case of rotating night shift workers. The transient and chronic disturbance of the circadian system is most frequently referred to as "circadian disruption", but many other terms have been proposed and used to refer to similar situations. It is now beyond doubt that the circadian system contributes to health and disease, emphasizing the need for clear terminology when describing challenges to the circadian system and their consequences. The goal of this review is to provide an overview of the terms used to describe disruption of the circadian system, discuss proposed quantifications of disruption in experimental and observational settings with a focus on human research, and highlight limitations and challenges of currently available tools. For circadian research to advance as a translational science, clear, operationalizable, and scalable quantifications of circadian disruption are key, as they will enable improved assessment and reproducibility of results, ideally ranging from mechanistic settings, including animal research, to large-scale randomized clinical trials.

Night shift work and risk of breast cancer in women: the Generations Study cohort

Background

It is plausible that night shift work could affect breast cancer risk, possibly by melatonin suppression or circadian clock disruption, but epidemiological evidence is inconclusive.

Methods

Using serial questionnaires from the Generations Study cohort, we estimated hazard ratios (HR) and 95% confidence intervals (95%CI) for breast cancer in relation to being a night shift worker within the last 10 years, adjusted for potential confounders.

Results

Among 102,869 women recruited in 2003–2014, median follow-up 9.5 years, 2059 developed invasive breast cancer. The HR in relation to night shift work was 1.00 (95%CI: 0.86–1.15). There was a significant trend with average hours of night work per week (P = 0.035), but no significantly raised risks for hours worked per night, nights worked per week, average hours worked per week, cumulative years of employment, cumulative hours, time since cessation, type of occupation, age starting night shift work, or age starting in relation to first pregnancy.

Conclusions

The lack of overall association, and no association with all but one measure of dose, duration, and intensity in our data, does not support an increased risk of breast cancer from night shift work in women.

Central regulation of breast cancer growth and metastasis

Cancer is a systemic disease. In order to fully understand it, we must take a holistic view on how cancer interacts with its host. The brain monitors and responds to natural and aberrant signals arriving from the periphery, particularly those of metabolic or immune origin. As has been well described, a hallmark of cancer is marked disruption of metabolic and inflammatory processes. Depending on the salience and timing of these inputs, the brain responds via neural and humoral routes to alter whole-body physiology. These responses have consequences for tumor growth and metastasis, directly influencing patient quality of life and subsequent mortality. Additionally, environmental inputs such as light, diet, and stress, can promote inappropriate neural activity that benefits cancer. Here, I discuss evidence for brain-tumor interactions, with special emphasis on subcortical neuromodulator neural populations, and potential ways of harnessing this cross-talk as a novel approach for cancer treatment.

An Overview of the Polymorphisms of Circadian Genes Associated With Endocrine Cancer

A major consequence of the world industrialized lifestyle is the increasing period of unnatural light in environments during the day and artificial lighting at night. This major change disrupts endogenous homeostasis with external circadian cues, which has been associated to higher risk of diseases affecting human health, mainly cancer among others. Circadian disruption promotes tumor development and accelerate its fast progression. The dysregulation mechanisms of circadian genes is greatly affected by the genetic variability of these genes. To date, several core circadian genes, also called circadian clock genes, have been identified, comprising the following: ARNTL, CLOCK, CRY1, CRY2, CSNK1E, NPAS2, NR1D1, NR1D2, PER1, PER2, PER3, RORA, and TIMELESS. The polymorphic variants of these circadian genes might contribute to an individual's risk to cancer. In this short review, we focused on clock circadian clock-related genes, major contributors of the susceptibility to endocrine-dependent cancers through affecting circadian clock, most likely affecting hormonal regulation. We examined polymorphisms affecting breast, prostate and ovarian carcinogenesis, in addition to pancreatic and thyroid cancer. Further study of the genetic composition in circadian clock-controlled tumors will be of great importance by establishing the foundation to discover novel genetic biomarkers for cancer prevention, prognosis and target therapies.