A growing number of studies have examined associations between night shift work and the risks of common cancers among women, with varying conclusions. We did a meta-analysis to identify whether long-term night shift work increased the risks of common cancers in women. We enrolled 61 articles involving 114,628 cases and 3,909,152 participants from Europe, North America, Asia, and Australia. Risk estimates were performed with a random-effect model or a fixed-effect model. Subgroup analyses and meta-regression analyses about breast cancer were conducted to explore possible sources of heterogeneity. In addition, we carried out a dose–response analysis to quantitatively estimate the accumulative effect of night shift work on the risk of breast cancer. A positive relationship was revealed between long-term night shift work and the risks of breast [OR = 1.316; 95% confidence interval (CI), 1.196–1.448], digestive system (OR = 1.177; 95% CI, 1.065–1.301), and skin cancer (OR = 1.408; 95% CI, 1.024–1.934). For every 5 years of night shift work, the risk of breast cancer in women was increased by 3.3% (OR = 1.033; 95% CI, 1.012–1.056). Concerning the group of nurses, long-term night shift work presented potential carcinogenic effect in breast cancer (OR = 1.577; 95% CI, 1.235–2.014), digestive system cancer (OR = 1.350; 95% CI, 1.030–1.770), and lung cancer (OR = 1.280; 95% CI, 1.070–1.531). This systematic review confirmed the positive association between night shift work and the risks of several common cancers in women. We identified that cancer risk of women increased with accumulating years of night shift work, which might help establish and implement effective measures to protect female night shifters. Cancer Epidemiol Biomarkers Prev; 27(1); 25–40. ©2018 AACR.

This article is featured in Highlights of This Issue, p. 1

In modern society, the fast-growing productivity demands for working across time zones and night shift work is increasingly prevalent in different industries such as food production, entertainment, health care, and transportation (1). Circadian disruption from electric lighting poses huge challenges on public health, including cardiovascular diseases, neuropsychiatric and endocrine system disorders, and even cancers, in particular breast cancer (2–4). Cancer incidence in industrialized regions is noticeably higher than that in developing countries, suggesting that environmental factors of modern society play an role in cancer etiology. In 2007, the International Agency for Research on Cancer (IARC) has identified “shift work that involves circadian disruption” to be probably carcinogenic (Group 2A), based on “limited evidence in humans for the carcinogenicity of shift work that involves night work,” and “sufficient evidence in experimental animals for the carcinogenicity of light during the daily dark period (biological night)” (5). Among night shift workers, female employees account for a large proportion, and the number of women in nursing is always overwhelming more than male. During 2004–2005, night shift workers accounted for 12.4% of the female working population and 17.4% for European countries (6). Long-term night shift work serves as a potential risk factor for the common cancers in female population.

Much of the research has examined that nocturnal melatonin suppression and circadian rhythm disruption caused by night shift work function as carcinogens that increase tumor incidence (5). Melatonin, primarily produced by pineal gland, was reported to play an important role in inhibiting tumor growth through antioxidation, antiangiogenesis, and regulation of immunity (7). Unnatural light at night reduced melatonin release, which contributed to tumor development. However, melatonin suppression had a negative feedback effect on hypothalamic-pituitary-gonadal axis, promoting gonadotropins secretion (8). Previous studies stated that night shift work increased the risk of hormone-dependent cancers including prostate cancer in men, breast cancer and ovarian cancer in women (9). In addition, another mechanism was related to clock genes expression which played an important role in several cellular processes such as DNA repair and cell apoptosis. Circadian rhythm disturbance among shift workers resulted in changes of clock genes expression, ultimately increased cancer susceptibility (10, 11).

Animal models focused on light at night to study the effect of circadian disruption on cancer incidence (12), whereas epidemiological evidence is limited with differing results. A growing number of females are being exposed to night shift work and employments in different working fields vary in exposure status such as frequency and period time. Nursing group are generally exposed to long-term and high-intensity night shift work. The association between night shift work and cancer risk of female nurses was not covered by published reviews. Breast cancer is the most common cancer in women worldwide. Most of previous meta-analyses emphasized, in particular, the association between night shift work and breast cancer and had controversial conclusions (13–17). Some of them concluded that night shift work was significantly associated with higher risk of breast cancer (13, 14, 17) whereas another study provided limited evidence (16) and also one study reported a small nonsignificant effect of long-term night shift work (15). Thus it still remains unknown whether night shift work elevates the risk of common cancers such as breast cancer, ovarian cancer and lung cancer. Here we performed a meta-analysis to demonstrate the effect of night shift work on the risk of common cancers among women. Meanwhile, the group of female nurses was evaluated for the separate risk estimation of multiple common cancers. By systematically integrating a multitude of previous data, we expected to arrive at a convincing conclusion which would help propose health protection programs for long-term female night shift workers.

Literature search

We conducted the meta-analysis following the quality standards of a meta-analysis. An extensive systematic literature search updated to October, 2016 was performed. We searched the keywords “night shift” “shift work” “shift-work” “cancer risk” “cancer mortality” in PubMed, Embase, Medline, and Web of Science databases. Only English articles were enrolled, and no other limitations were restricted. Also, we manually searched citing and reference lists to identify other relevant studies.

Inclusion and exclusion criteria

Studies were identified according to inclusion and exclusion criteria. Studies were included if they met the following criteria: (i) cohort study, case–control study, or nested case–control study within cohort study; (ii) study evaluating cancer risk among women that were ≥18 years old and were exposed to night shift work; (iii) study involving OR, RR, HR, or standardized incidence ratio (SIR) with 95% confidence intervals (CIs) or providing sufficient data to calculate the above parameters. Studies were excluded if they met the following criteria: (i) study involving female cancer risks that could not be separated from that of male; (ii) study providing overlapping or insufficient data; (iii) study involving recurrent cancer.

Data extraction

Extracted information from enrolled studies included first author, published year, number of cases and subjects, OR and corresponding 95% CI, study design, quality score, region, type of cancer, range of night shift work, occupation, variables of adjustment, and exposure assessment. We adopted the shortest and longest exposure time for preceding analysis. Data extraction was performed independently by two investigators and a third author resolved the differences by face-to-face discussion.

Quality evaluation

We used the Newcastle-Ottawa Quality Assessment Scale (NOS; ref. 18) for quality evaluation of eligible studies. NOS adopted the star system with a maximum of nine stars scoring from 0 to 9, which was divided into four parts: participant selection, comparability of study group, exposure assessment, outcome evaluation, and scoring <7 indicated a low quality. NOS quality evaluation was conducted by two investigators independently and a third author settled all disagreements.

Statistical analysis

We evaluated the association between night shift work and female cancer risk using statistical software STATA Version 11.0 (StataCorp). ORs with their corresponding 95% CIs were used as effect measure. Statistical heterogeneity was evaluated by Q and I2 statistics. P < 0.10 and I2 > 50% indicated an existence of statistical heterogeneity and a random-effect model was then carried out, otherwise a fixed-effect model was used (19). To explore the possible sources of heterogeneity regarding breast cancer, meta-regression analyses were performed. Moreover, we conducted subgroup analyses stratified by region, study design, occupation, exposure assessment, number of variables, and quality score.

For dose–response meta-analysis, we retrieved studies which involved at least three levels of exposure categories and information of cases, number of total subjects, person-year, years of exposure in each category were extracted. The midpoint of lower and high boundary was used as average time of night shift exposure. The range of highest category was supposed to be the same as the adjacent category if the upper boundary was not provided (20). Two-stage random-effect model was adopted to estimate the overall dose–response trend.

Begg funnel plot was performed to evaluate the publication bias of enrolled studies and P < 0.05 suggested the evidence of publication bias (15). In the Begg funnel plot, the standard error of logarithm (Log) for OR was plotted against its OR, and Log OR was plotted versus standard error of Log OR for each enrolled study (21).

Literature search and selection of studies

The initial search yielded 368 articles, and 98 articles were retrieved after checking titles and abstracts. Then, we reviewed full texts of these articles and 56 were included according to eligibility criteria. Also, 5 relevant studies were identified by manually searching citing and reference lists. Finally, 61 articles (22–82) were eligible for a comprehensive analysis. The selection flowchart is shown in Fig. 1.

Study characteristics

Sixty-one articles were enrolled in the meta-analysis including 26 cohort studies, 24 case–control studies, and 11 nested case–control studies. Several articles investigated whether the carcinogenic effect of night shift was related to breast cancer estrogen-receptor status (ER+ and ER breast cancer; ref. 38) or menopausal status (premenopausal and postmenopausal; ref. 46) or different years in employment (60). In addition, several articles reported the risks for a number of cancers including breast cancer, cervical cancer, and colon cancer, and so on (57, 61, 62, 71, 77). One study (82) evaluated associations between night shift work and the risk of breast cancer in two cohorts, respectively [Nurses' Health Study (NHS) and Nurses' Health Study II (NHS2)]. Thus, 67 studies from 61 articles comprising 114,628 cases and 3,909,152 participants were analyzed for the association between night shift work and common cancers at 11 sites of women including breast cancer, digestive system cancer, skin cancer, reproductive system cancer, hematologic system cancer, endocrine system cancer, nervous system cancer, urinary system cancer, lung cancer, bone and soft tissue cancer. Most studies reached a standard of high follow-up rate or favorable response rate. Seventeen out of 67 studies were conducted among nurses, and 49 out of 67 studies evaluated the association between night shift work and the risk of breast cancer. Questionnaire, interview, and databases were adopted for exposure assessment, and we extracted the shortest versus longest history of night shift work as exposure indicator. Forty-nine studies were adjusted for >3 confounders and 18 studies for ≤3 confounders. The main characteristics of the included studies were summarized in Table 1.

Risk assessment and heterogeneity

We integrated multivariable-adjusted ORs of longest versus shortest exposure duration to identify the correlation between night shift work and the risk of common cancers. Long-term night shift work increased the risk of female cancer (OR = 1.190; 1.122–1.262) and OR forest plots (with 95% CIs) of cancer for long-term night shift female workers were shown in Supplementary Fig. S1. A positive association was observed regarding breast cancer (OR = 1.316; 95% CI, 1.196–1.448), digestive system cancer (OR = 1.177; 95% CI, 1.065–1.301) and skin cancer (OR = 1.408; 95% CI, 1.024–1.934). OR forest plots (with 95% CIs) of breast cancer, reproductive system cancers, digestive system cancers, lung cancer, and skin cancer were shown in Fig. 2. ORs of association between night shift work and the risk of common cancers in women were summarized in Supplementary Table S1. With regard to nurses, the risk of six common cancers were estimated and as a result night shift work was associated with increased risk of breast cancer (OR = 1.577; 95% CI, 1.235–2.014), digestive system cancer (OR = 1.350; 95% CI, 1.030–1.770), and lung cancer (OR = 1.280; 95% CI, 1.070–1.531). ORs of the common cancers in nurses were shown in Supplementary Table S2.

Significant heterogeneity was observed in the groups of breast cancer (P = 0.000, I2 = 80.4%), skin cancer (P = 0.009, I2 = 64.7%), and uterine cancer (P = 0.042, I2 = 59.6%), and risk estimates were conducted with a random-effect model. No evidence of heterogeneity existed among the other groups.

Subgroup analysis and meta-regression analysis

Subgroup analyses about breast cancer were conducted stratified by geographic location, study design, number of variables, study quality, exposure assessment, and occupation. When stratified by geographical location, Europe (OR = 1.416; 95% CI, 1.246–1.610) and North America (OR = 1.236; 95% CI, 1.048–1.459) shared higher OR estimates, whereas positive correlation did not exist in Asia (OR = 1.236; 95% CI, 0.865–1.767) and Australia (OR = 1.040; 95% CI, 0.852–1.271). When stratified by design, the highest pooled OR estimate was obtained among nested case–control studies (OR = 1.555; 95% CI, 1.115–2.169) and the lowest was among cohort studies (OR = 1.193; 95% CI, 1.030–1.382). In terms of occupation, it was revealed that the strongest OR existed in the nursing group (OR = 1.577; 1.235–2.014). In the analysis stratified by quality score, both groups of high score (OR = 1.273; 95% CI, 1.152–1.407) and low score (OR = 1.434; 95% CI, 1.133–1.814) presented a positive association but the latter tended to have significant heterogeneity (P = 0.000, I2 = 89.7%). As for the number of adjusted confounders, both groups indicated that night shift work was associated with increased risk of breast cancer and significant heterogeneity (P = 0.000, I2 = 89.1%) existed in the group adjusted for ≤3 confounders. When stratified by exposure assessment method, the questionnaire group presented a stronger correlation and lower heterogeneity compared with interview and database group, whereas negative association was observed among studies adopting other assessment methods (OR = 0.906; 95% CI, 0.596–1.378).

Furthermore, meta-regression analysis was carried out to explore the possible heterogeneity sources among stratified factors and none of the variables was considered as a potential source of heterogeneity. The results of subgroup analyses and meta-regression were summarized in Table 2.

Dose–response meta-analysis

Dose–response meta-analysis was performed among studies that involved at least 3 levels of exposure categories. The number of studies on breast cancer accounted for an overwhelming proportion and sixteen studies were included to quantitatively assess the cumulative effect of exposure to night shift work on breast cancer incidence. As for other types of cancers, the number of relevant literature was not enough for dose–response meta-analysis. For every 5 years of night shift work, the risk of breast cancer in women increased by 3.3% (OR = 1.033; 95% CI, 1.012–1.056) as shown in Fig. 3.

Study quality

The NOS was employed for quality evaluation of eligible studies. Their scores ranged from 5 to 8 and the mean value was 7.1, indicating a favorable overall quality. Among the included studies, 43 articles were considered to be of high quality with scores ≥7 and 8 studies got 5 scores because of lack of complete research records and variables of adjustment.

Publication bias

We performed Begg funnel plot to assess the publication bias of included studies. Potential publication bias was identified (P = 0.006) among all retrieved studies probably due to the variety of involved cancers, whereas no publication bias was observed among studies on breast cancer (P = 0.208; Supplementary Fig. S2).

Some people in contemporary society work in a 24-hour mode, disrupting the 8-hour day routine (83). With the productive and economic development, night shift work is strongly required in the fields of industry, commerce, and entertainment. Shift workers suffer from disturbance of circadian rhythm and suppression of nocturnal melatonin. Short-term effects of night shift work were summarized as “jet-lag” syndrome, including sleep disorders, digestive troubles, fatigue, emotional fluctuation, and reduced physical activity. Long-term night shift work was reported to be associated with increased risks of cardiovascular disease, neuropsychiatric disorder, endocrine system disorders, and cancer (84–87). Data from the third EU Survey (2000) showed that 76% employee worked beyond normal working time (88). Up to 21.9% of men and 10.7% of women were exposed to shift work, with 7% population working permanently at night (89). Large numbers of people are being exposed to night shift work, which brings huge detrimental impact on health; it is therefore of much significance to conduct the study to illustrate the relationship between night shift work and the risks of frequently-occurred cancers in women.

Sixty-one studies were enrolled in the meta-analysis including 26 cohort studies, 24 case–control studies, and 11 nested case–control studies with 114,628 cases and 3,909,152 participants from Europe, North America, Asia, and Australia. A positive dose–response relationship was present between night shift work and the risks of breast cancer (OR = 1.316; 95% CI, 1.196–1.448), digestive system cancer (OR = 1.177; 95% CI, 1.065–1.301), and skin cancer (OR = 1.408; 95% CI, 1.024–1.934). Among the group of nurses, long-term night shift work increased the risks of breast cancer (OR = 1.577; 95% CI, 1.235–2.014), digestive system cancer (OR = 1.350; 95% CI, 1.030–1.770), and lung cancer (OR = 1.280; 95% CI, 1.070–1.531). A nonsignificant effect was observed for ovarian cancer (OR = 1.135; 0.970–1.328) and no effect was seen for cervical cancer (OR = 0.980; 95% CI, 0.787–1.221). Night shift work elevated the risk of breast cancer in a dose–response way which was consistent with previous studies (6, 20, 90). For every 5 years of night shift work, the risk of breast cancer in women increased by 3.3% (OR = 1.033; 95% CI, 1.012–1.056).

Night shift work causes an increase in sex hormones, which is speculated to be relevant for hormone-dependent cancers (91). Strong epidemiologic evidence supports the association between night shift work and increased risk of breast cancer, and also there is limited evidence on prostate cancer (92, 93) and endometrial cancer (25). In our analysis, the risks of hormone-sensitive cancers including breast cancer, ovarian cancer, and uterine cancer among night shift workers were shown in Table 2. Night shift work was strongly associated with higher risk of breast cancer in females whereas no effect was observed for ovarian cancer and uterine cancer.

The underlying biological mechanisms of the association between night shift work and increased cancer risks are complex. One of the possible hypotheses is that exposure to light at night accompanying night shift work results in the disruption of circadian rhythm and the reduction of melatonin production (5). Melatonin is characterized with oncostatic effect which works through antioxidation, antiangiogenesis, regulation of immunity, and metabolism (7). Melatonin reduction stimulates the production of pituitary gonadotropins by negative feedback, hence increasing the risk of sensitive cancers such as breast cancer, ovarian cancer, and endometrial cancer (8). Animal experiments have demonstrated that long-term oral melatonin supplement offered a protective effect against breast cancer (94). Several in vitro researches showed that melatonin administration of even biological dose had a significant growth inhibition effect on breast cancer cells (95–98) and other tumor cells (99–103). Sun exposure is sharply decreased among night shift workers, leading to reduced vitamin D level. Some experimental and epidemiological researches supported the inverse correlation between circulating vitamin D and the risk of breast cancer (104, 105) or colorectal cancer (106, 107). However, night shift work was often accompanied with irregular eating habits, which somewhat contributed to digestive system tumors. Results in our study were inconsistent with the above theories that night shift work increased the risk of hormone-sensitive cancers and digestive system cancers.

Q and I2 statistics were used to evaluate heterogeneity. As a result, significant heterogeneity was observed in the groups of breast cancer (P = 0.000, I2 = 80.4%), skin cancer (P = 0.009, I2 = 64.7%), and uterine cancer (P = 0.042, I2 = 59.6%). Random-effect model was adopted in an attempt to eliminate all sources of heterogeneity. Furthermore, subgroup analyses about breast cancer were conducted and less heterogeneity existed among cohort studies, high-score studies and studies adjusted for >3 confounders, suggesting that these studies would provide more reliable evidence. Moreover, we carried out meta-regression analysis and no explanation was found for possible heterogeneity sources from variables due to low statistical power, therefore the results of risk estimates should be interpreted with caution. When stratified by region, positive correlation existed in Europe and North America, but not in Asia and Australia. One possible reason might be that Asian population was less sensitive to nightshift exposure. Another was partly attributed to the difference of sleeping habits, economic development, and medical service across different geographical areas. Short sleep duration, light at night, and airline cabin crew servings involved potential circadian rhythm disturbances just like nightshift; therefore, relevant articles were included in this meta-analysis to reduce selection bias. Flight attendants were simultaneously exposed to cosmic radiation which was a potential cancer-related unmeasurable factor, thus the odds risk might be overestimated. Among nurses, remarkable elevation of cancer risk was observed regarding breast cancer (OR = 1.577; 95% CI, 1.235–2.014), digestive system cancer (OR = 1.350; 95% CI, 1.030–1.770), and lung cancer (OR = 1.280; 95% CI, 1.070–1.531). In the future, large-sample and multiregion researches are needed to update and confirm the association.

The present meta-analysis involved 3,909,152 participants and 114,628 female patients with cancers at 11 sites. To the best of our knowledge, this is the first meta-analysis to comprehensively assess the association between night shift work and the risk of common cancers among female population. Compared with previous meta-analyses, the study has the following merits. First, in the methodological aspect we systematically conducted risk estimates, subgroup analyses, meta-regression analysis, and dose–response meta-analysis. Random effect model was used to eliminate the source of heterogeneity to some extent. In addition, subgroup analyses and meta-regression were performed to explore potential sources of heterogeneity from confounding factors. Second, according to inclusion and exclusion criteria, 61 studies were enrolled in the meta-analysis and the accumulated evidence with enlarged sample size enhanced statistical power to derive a more precise and reliable risk estimation. Beyond that, longest versus shortest duration was taken as exposure indicator and each individual article was involved in the pooled risk estimate, increasing the generalizability of results. Third, the majority of included studies (40 out of 67) were carried out among general population, therefore the estimation of association between night shift work and cancer risk of women could be extended, not limited to some particular working groups. A certain amount of studies (17 out of 67) were based on nursing group which was an important part of female shift workers and a stratified analysis revealed that night shift work increased the risk of breast cancer, lung cancer and digestive system cancer in female nurses. Fourth, cohort study is less susceptible to confounding factors and less affected by recall bias, thus result from cohort study is considered more credible and valuable compared with case–control study. In this meta-analysis, 26 cohort studies with enough follow-up period were included and a positive relationship was found in the cohort group.

Nevertheless, some shortcomings in the study have to be mentioned. First, the period time of night shift across all enrolled studies was not defined uniformly, for example one definition was “working at least three nights per month” (29) and another was “working during the night (23:00–06:00 h)” (28). Included studies used very different definitions of period time regarding night shift and the lack of consistent definition might result in a certain degree of misclassification, consequently leading to a dilution of pooled estimates when doing data synthesis. Second, a significant between-study heterogeneity and publication bias was observed. Significant variability existed in different individual studies regarding study population, geographical location, adjustment confounders, study design and exposure definition, and each of these factors may contribute to heterogeneity. Disappointingly, we failed to find out the possible sources of heterogeneity from a meta-regression due to the low statistical power. Given substantial heterogeneity observed among included studies, the evidence supporting the association might be weakened and additional well-designed researches are needed. Third, case–control study are vulnerable to varying levels of bias, and in general patients in case group tend to recall past exposure history of night shift. Twenty-four case–control studies were included in our analysis which could probably bring about selection and recall bias. Also, unstandardized questionnaires might produce information bias and inadequate control of adjustment factors could lead to either underestimation or exaggeration of the pooled risk. Finally, only a small number of enrolled studies were involved in dose-response meta-analysis, hence limiting the reliability of the result.

In conclusion, this meta-analysis updated previous studies and identified that cancer risk in female population was increased with accumulating years of night shift work. Given the expanding prevalence of shift work worldwide and heavy public burden of cancers, further researches, particularly large-size, high-score cohort studies are of great necessity to confirm the relationship between night shift work and cancer risk. Also, in-depth biological researches should be done to explore the mechanisms by which night shift work affects cancer risk. Knowing how night shift work serves as a risk factor for cancers might help establish and implement effective measures to protect female night shifters. Moreover, it is important that long-term night shift workers accept regular physical examination and cancer screening for potential malignancies, particularly breast cancer.

No potential conflicts of interest were disclosed.

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