Relationship between Night Shifts and Risk of Breast Cancer among Nurses: A Systematic Review

Background and objectives: The incidence of breast cancer worldwide has increased in recent decades, accounting for 1 in 3 neoplasms in women. Besides, nurses are mainly represented by the female collective, most of them, undertaking working conditions with intensive rotative and night shifts due to the 24-h pace of work of this profession. The objective of this study was to assess the possible relationship between shift work, especially night-time work, and the development of breast cancer among nurses. Materials and Methods: A systematic review of the literature was carried out through the consultation of the following databases: Cochrane Plus Library, PubMed, CINAHL (Cumulative Index to Nursing and Allied Health Literature), Web of Science, ScienceDirect, Scopus, and Dialnet. Records were selected between 2010 and 2020, in Spanish and English, which covered the association between breast cancer diagnosed among nursing professionals and rotating night shifts. Results: A total of 12 studies were identified after critical reading. Most of the studies found an association between breast cancer and consecutive rotating night shifts prolonged over time. Among the associated factors, the alteration of the circadian rhythm influenced the expression of peripheral clock genes, which was the same as reproductive hormones. The risk of breast cancer in nurses increased during early adulthood and after 5 or more years with 6 or more consecutive nights. Conclusions: The different studies of this review show significant associations between breast cancer and prolonged rotating night shifts. Similarly, there is a relationship between the alterations in certain circadian rhythm markers (such as melatonin), epigenetic markers (such as telomeres), and breast cancer that would require more studies in order to corroborate these findings.


Introduction
Breast cancer currently represents the neoplasm that causes the highest incidence and death among women. This type of neoplasms usually presents itself as a painless mass in one or both breasts, where the help of complementary exploratory tests is necessary for diagnosis, such as breast imaging tests (mammography) as proof of choice, or breast biopsies if the image suggests malignancy, which allows a histological diagnosis and an immune-histochemistry study. Its prognosis is related to the biological subtype and tumor stage of the patient. Treatments will vary depending on the characteristics and screening, the FLC 3.0 web platform [19] is utilized for the critical review of the chosen references. This revision protocol has been recorded in PROSPERO with registration number CRD42020216309. Based on the information detailed in Table 1, the following question was asked: "Is there a link between breast cancer and night work among nurses?"

Databases and Documentary Sources Consulted
The bibliographic search took place between 7 July 2020 and 27 July 2020. This review was carried out in the following databases:

Keywords
In the search, keywords obtained from the Descriptors in Health Sciences (DeCS) and Medical Subject Headings (MeSH), developed by the National Library of Medicine (NLM), were used depending on the language used in the database. Details of the different search strategies used are displayed in Table S1. The keywords included in the search terms are detailed by language in the following table (Table 2):

Inclusion and Exclusion Criteria
The inclusion and exclusion criteria were as follows: Inclusion criteria: • Studies carried out over the last 10 years (2010-2020).

•
In Spanish and English. • Peer-reviewed articles. • Typology: original articles and clinical trials, systematic meta-analysis and reviews, short/brief communication, and case reporting.

•
They should analyze at least one of the following characteristics on a nursing sample (population): risk factors related to shift work among nursing professionals and breast cancer, associated hormonal changes and/or alterations after blood sample, light levels to which nurses are exposed at night work, and circadian gene expression.
Exclusion criteria: • Records of low scientific evidence.

•
Articles that have no relation to the purpose of the review. • Typology: opinion articles, editorials, and letters to the director/editor.

Critical Appraisal and Level of Evidence
For the critical appraisal of the bibliography, the FLC 3.0 web platform for Critical Reading Sheets (Osteba: Pais Vasco, Spain) [19] was used, a tool for assessing study quality developed by the Health Technology Assessment Service of the Basque Government. This tool uses the recognized evaluation criteria according to the study design that evaluates each sheet, and thus assesses the methodological quality of each study and the degree of bias in the different methodological designs, facilitating criteria homogeneity among reviewers. Both the quality assessment and data extraction were independently carried out between pairs, and a third party acted as an evaluator comparing the information collected between both evaluators and agreeing on the contents of the final template.
The selected records were assessed following the following points below: Study design, objective, location, and period of completion. Once this section had been complied with, the level of quality of scientific evidence of each of the studies was subjectively determined, and a classification with a low, medium or high level was be obtained. After this process, studies that obtained a low-quality level of evidence were excluded (Table 4). Not assessable: Having answered 'No information' in the "Method" area or in "Majority of areas", so assessing study quality is not possible

Reverse Search
As a secondary strategy, a search was carried out using references and author names cited in the different selected records (reverse or snowball search), with the intention of verifying the existence of works not found in the primary search.

Records' Selection
The total number of records found in the different databases was 371, of which 37 were selected after a first screening where studies that did not provide relevant data to the work, after reading the title and summary, were discarded. Of these, 25 studies were kept after the removal of duplicates. Once the evaluation of these studies was carried out, those that met the pre-established inclusion criteria were selected, with a total of 12 records meeting these requirements (see Table 3). Following this process, and as the last phase of selection, these 12 studies met the quality criteria [19] established in the critical reading phase [2,[4][5][6][7][8][9][13][14][15]21,29] (Table 4). The flowchart for this study is presented in Figure 1.    Table 5 [2,[4][5][6][7][8][9][13][14][15]21,29] summarizes the evaluated aspects of the 12 selected studies: main study characteristics, aim of the study, intervention and instrument, main findings, conclusions, and quality assessment for each of the records. Nurses who worked at night had a significantly longer working life, were younger at menarche and menopause, had fewer children, were older at the birth of their first child, spent fewer hours weekly on sport, and had slept fewer hours per night. Day-evening-night shiftwork is the most frequent rotating shift system. There is a tendency to increasing odds ratios for breast cancer by cumulative years of shiftwork and shift systems that disrupt circadian rhythms (i.e., night shifts and rotating day-night shifts). The most frequent working schedule was 6-7-night duties per month. There was no significant difference in the circulating sex hormone concentrations between current night shift workers and day workers. There was significant association between total duration of night work (>15 years) and higher estradiol levels among postmenopausal women. No significant associations were found with night work among premenopausal women, although the mean concentration of hormones is higher among women with longer night shift duration. DNA methylation of oestrogen receptor genes (ESR1, ESR2) play a significant role in the proliferation of breast tissue stimulated by estrogens, which is a known as breast cancer risk factor. Reduced telomere length is found in nurses with at least 12 years of night shifts.

Discussion
The association between breast cancer and rotating night shifts among nurses is a constant in most of the examined studies in this review. The majority of studies reported a relation between increased breast cancer risk and cumulative years working in night shifts, normally with 3 or more nights per month for 15 years or more [2,[4][5][6][7][8][9][13][14][15]29]. Risk increases in permanent night shifts and long-term day-night rotating shifts [2]. A long duration of shift work throughout years is also related with estrogen and progesterone positive tumors, mostly found among young women with intensive shifts (12 h-shifts) [8,9,14,15,21]. Most of the studies add information that relates rotating shift work and lifestyle factors [2,[4][5][6][7][8][9][13][14][15]21,29], being quality of sleep, obesity, diabetes, early menopause, number of childbirths, and hormonal treatment the most common factors related with night work. However, the relationship between rotating night work and the expression of circadian genes [5,6,8,13,21], as well as the relationship between certain markers of circadian rhythm and genetic alteration or expression [7,13,21,29], continues to be discussed.
Wegrzyn et al. [14] examined, on two prospective cohorts, Nurses' Health Study (NHS: 1988-2012, sample: 78,516), and Nurses' Health Study II (NHS2: 1989-2013, sample: 114,559), with 9541 cases of invasive breast malignancies and 24 years of follow-up. In the NHS, women with more than 30 years of shift work did not have an increased risk of breast cancer (Risk Ratio (RR)-0.95 (95%); CI 95% = 0.77-1.17; p = 0.63), as compared to those who never worked in shifts, although the follow-up was mainly carried out after the women finished doing shift work. However, at NHS2, there was a significant increase in the risk of breast cancer among nurses with more than 20 years of work, reflecting exposure among young adult nurses (RR 2.15; CI 95% = 1.23-3.73; p = 0.23) and significantly increasing among those nurses who had been performing cumulative rotating night shifts for more than 20 years since their early career, updating the exposure information (RR 1.40; CI 95%, 1.00-1.97; p = 0.74). Thus, associations between rotating night shifts work and the risk of breast cancer concluded that rotating night work, sustained over time, increased the risk of breast cancer among those women who performed these shifts at an early age.
The study conducted by Lie et al. [6] in 2011 on a cohort of 49,402 Norwegian nurses found a total of 699 (74%) of cases diagnosed between 1990-2007, and alive at the moment of the study, and 895 (65%) cancer-free cases at the time of the sampling. This study assessed the association between breast cancer and night work, and suggested that these results could establish a link between the risk of breast cancer and the number of consecutive shifts, finding a significantly higher increase in breast cancer risk, between 10-30%, among those professionals with a higher level of exposure and establishing that those nurses who had worked 5 years with 6 consecutive night shifts (OR 1.8; 95% CI: 1.1-2.8) had an increased risk of developing the disease, as compared to those who had never worked on the night shift. In later studies, a number of polymorphisms was found in circadian genes and melatonin biosynthesis that could be associated with the risk of breast cancer among nurses working on shifts [30,31]. Hansen et al. [4], in another case and control study, conducted in 2011 based on interviews to a national cohort of 91,140 women, members of the Danish Association of Nurses (95% of nurses in Denmark), also exposed an increasing trend in the increase in cases of breast cancer related to cumulative shifts work with greater or lesser risk to alter the circadian rhythm.
Although the number of studies included in the systematic review conducted by Dickerman et al. [2] in 2012 was somewhat limited, its authors also explained that exposure to light during periods of night work may contribute to an increased risk of breast cancer among nurses, analyzing potential action mechanisms associated with night work such as melatonin suppression, sleep disruption, lifestyle factors (poor diet and exercise, higher body mass index) or lower levels of vitamin D. Similarly, the characteristics of night work were significantly associated with alterations in bodily hormones such as oestradiol [9,21] or insulin [15]. Some studies have strengthened the relationship between urinary levels of 6-sulfatoxymelatonin (aMT6s) and shift work, a stable metabolite of urinary melatonin that appears to be related to breast cancer [15,28]. Moreover, the release of luteinizing hormone (LH) and follicle stimulating hormone (FSH) by the pineal gland, which stimulates the production and release of oestrogen by the ovaries, was associated with melatonin levels, so it can be concluded that the decrease in circulating melatonin concentration could give rise to a greater amount of oestrogen secreted by the ovaries [15]. For its part, the case and control study carried out by Lie et al. [8] in 2013 assessed the effect of night work on diagnosed breast cancer nurses between 1996-2007 on 757 controls that were selected from a nurse cohort in Norway. The study offered statistically significant data on the impact that long-term night work (≥5 years) with 6 or more consecutive shifts have on the increased risk of breast cancer, noting significant associations and an increased risk for tumors with positive progesterone receptors (OR = 2.4, CI 95%: 1.3-4.3; p = 0.01). These findings suggested an important effect of progesterone on the negative impact that shift work has among nursing professionals. In line with this finding, the Carugno et al. [29] study observed a reduction in methylation of the ESR1 gene (mediated by linking progesterone to the promoter region) associated with night shift work, both by analyzing current night shift workers versus non-night shift workers, and by comparing those who never worked on the night shift, suggesting that low levels of ESR1 were associated with increased proliferation of breast tissue.
Erdem et al. [7], in his 2019 study, referred that telomere length (TL) shortening was associated with an increased risk of breast cancer among workers who had performed long periods of consecutive night shifts, evaluating quantitative polymerase chain reaction (qPCR) in the DNA of 563 patients diagnosed with breast cancer and 619 Norwegian control nurses. The data indicated that TL was not significantly different among nurses who had worked night shifts as compared to those who had worked only on isolated days, concluding that the duration of night work, whichever the night shift intensity, did not influence TL. On the contrary, it was noted that working many consecutive night shifts for at least 5 years had a correlation with a reduction in TL regardless of the state of the control cases. Another study based on the genetic study concluded the same when working night shifts for at least 12 years [29]. These data suggest that shortened TL is affected by night work schedules and could be a contributing factor to the risk of breast cancer among women working shifts.
The systematic review carried out by Rosa et al. [15] in 2019 showed data suggesting a link between night shift work and disruption of sleep rhythm among nurses, noting that some of the studies showed that sleep reduction was associated with an increased risk of breast cancer, and also finding an association between breast cancer, shift work, and the number of consecutive night shifts performed over a period of more than 1 month. In this case, the risk of breast cancer was significantly higher (OR = 1.8; CI 95% = 1.2-2.8; p = < 0.05) among nursing professionals who had worked for more than 5 years with 6 continued rounds on night shifts. It was stated that the dehydration effects caused by night work were derived from the secretion of cortisol and that, in addition, the functioning of the hypothalamic-pituitary-adrenal axis was affected by lack of sleep hours and deficit daytime rest, contributing to the mismatch of the immune system and impacting on the effectiveness of the anti-tumor surveillance system [15]. It was noted that nurses working night shifts for a period of 1 to 29 years had an 8% risk of developing breast cancer. Meanwhile, nurses who had worked night shifts for more than 30 years showed a higher risk of up to 36%. Among the signs of circadian rhythm disturbance, the review by Rosa et al. [15] also found an association between the increase in irregular menstrual cycles and night shift workers, this being considered a risk factor for breast cancer. Similarly, the review carried out by Vega-Escaño et al. [32] described shift work (regardless of its characteristics and context) with a direct relationship for the onset of insomnia, which could lead workers to be extremely exposed to risks arising from their professional activity, in addition to other factors such as stress, self-medication, tobacco abuse, or the use of psychoactive substances.
Some limitations found in this study were as follows: (a) having used specific search tools (Cochrane, PubMed, CINAHL, Web of Science, Science Direct, Scopus, and Dialnet), so there is a possibility of lost research elements; (b) studies written in English and Spanish from 2010 to the present day were taken into account, excluding the possibility of prior research or research in other languages; (c) some research had a small number of participants and did not meet the objectives of this study after the first critical reading; (d) despite avoiding article selection bias, in order to focus the analysis on breast cancer and shift work schedules among nurses, the inclusion and exclusion criteria limited the review field by excluding low-quality research, other cancers or other professionals; (e) in most of the analyzed research, there are many confounding characteristics among the samples being analyzed, so this should be considered generalizing the findings to the entire nursing population; (f) performing a meta-analysis was not established as an objective of this study. Moreover, heterogeneity has been observed among the selected studies in terms of the analyzed variables, sample size, definition of shiftwork, and analytic methods, so meta-analysis was not employed in a further step.
The results of the study can be valid, indirectly, for all night shift workers apart from healthcare. This is, industrial, transport, communications, leisure, and hospitality sectors. As future measures, analyzing breast cancer risk factors as compared with occupational factors would be necessary to correlate the molecular mechanisms of cells' circadian control, specifically in the event of circadian disruption during work schedules. For this, a multivariate examination with regression models is projected to understand the entire complexity of shift work, in which the role of moderating/mediating factors remains unexplored. This analysis would be useful to propose preventive measures such as a healthy methods of shift rotation (mostly with reduction of night shifts), developing labor risk stratification algorithms, or assessing the behavioral consequences of different types of rotation.

Conclusions
The different studies of this review showed a significant relation between breast cancer and prolonged rotating night shifts in the established time. In this way, cumulative years working at night, long shift length (12 h), and performing more than 6 night shifts per month for at least 5 years or more are found as a potential breast cancer risk factors, especially in hormone-dependent cancers and among nurses who started working at night in their early career. Similarly, there is a relationship between alterations in certain markers of circadian rhythm such as melatonin or in markers of epigenetic alteration such as telomeres length and breast cancer, that would require further studies in order to support these findings.
Today's world has an increasing and faster trend towards the so-called "24-h societies". To this we must add the need for continuous and necessary care that patients require, so it would be beneficial to apply preventive measures that minimize or avoid as much as possible these alterations in order to reduce the incidence of breast cancer among nurses.

Conflicts of Interest:
The authors declare no conflict of interest.