Background: It has been found that night work might increase the risk of breast cancer by exposure to light at night with subsequent melatonin suppression. Objective: To assess the association between night work and the risk of developing breast carcinoma. Methods: A literature review was conducted regarding breast cancer, night work, melatonin suppression and night light exposure. Results: A decrease in melatonin may increase the risk of breast cancer as a result of its protective abilities on regulating the immune system and on anti-mitosis actions among other protective abilities. The overall evidence from the studies examined indicates that frequent night work increases the risk for breast cancer. The risk is greater in women who worked for 20 years or more on night shifts. Conclusion: This research confirms that the increase of years working night shifts corresponds to an increase in the risk of breast cancer. Abstract Breast cancer is the leading type of cancer in women, with 1.38 million new diagnoses in 2008 globally [1]. One of the possible risk factors for breast cancer is night shift work. In 2007, the International Agency for Research on Cancer categorized night shift work as a probable human carcinogen [2]. It is hypothesized that night shift work is a possible risk for breast cancer as it disrupts the normal circadian rhythms. While the biological mechanism linking night shift work with cancer risk remains unknown, the main hypothesis pathway involves melatonin [3]. Melatonin, a pineal hormone, is closely linked to the circadian system and demonstrates cancer- protective capabilities [4]. Increased use of artificial light (in the evening, night, or early morning) is thought to decrease production of melatonin, which may induce continuous production of estrogen, involved in breast carcinogenesis and therefore, increasing cancer risk [5]. The purpose of this research was therefore to determine the relationship between night shift work and the risk of breast cancer. Introduction Literature review using scientific databases: Pubmed/Medline, Scopus and Web of Science Search items included: breast cancer or breast carcinoma as a keyword combined with night shift terms, occupations terms and work terms. Inclusion criteria: English studies published between 2000 and 2013, that focused on night shift work, the risk of breast cancer and females aged 20 to 80 years. Eligible studies also had reported results in the form of odds ratio, relative risk and/or hazard risk with their corresponding 95% confidence intervals. Exclusion criteria: Studies that were excluded were those that focused on animals or did not separate men from women in results Methodology Results Schernhammer et al. (2001) observed increased risk with increasing numbers of years in night shift. The relative risk (RR) associated with 30 or more years of rotating night work. Schernhammer et al. (2006) observed RR associated with extended periods (20 or more years) of rotating night shift work 1.79 (95% CI, ). Tynes et al. (1996) found a RR for breast cancer of 1.5 (95% CI, ). Hansen (2001) found a RR for breast cancer among women who had worked at night (≥ 60%) for at least half a year was 1.5 (95% CI, ) Lie et al. (2011). Significantly increased risks were seen in nurses who worked more than 5 years with 6 consecutive night shifts. Lie et al. (2013) observed an association between breast cancer and work durations of more than 5 years with more than 6 consecutive night shifts. Hansen and Lassen (2012) found that frequent night shift work increases the risk for breast cancer and suggests a higher risk with longer duration of night shifts. More importantly, an OR of 3.9 [95% CI, ] was observed in women with morning chronotype preference and intense night shifts. Davis et al. (2001) observed a RR of breast cancer of 2.3 (95% CI, ) for 5.7+ h per week of graveyard shift. Pesch et al. found in their study that having ever done night work was not associated with an elevated breast cancer risk. Grundy et al. (2013), found that associations with duration demonstrated association was apparent for over 30 years. Manegaux et al. (2012) observed an OR of 1.95 (95% CI, ) in women employed in night work for over 4 years before their first full-term pregnancy. Results continued Through the evaluation of the studies included, a statistically significant influence of night work on breast cancer was found. An important factor in the association between the risk of breast cancer and night shift work was the number of nights consecutively worked as well as the number of years employed in night shift work. However, not all studies agree with these results, many finding inconclusive results, such as the study by Pronk et al. (2010), which did not find any evidence of effect modification, even under long duration. However, a meta- analysis by Wang et al. (2013) discovered a positive dose-response relationship between night work and breast cancer with increasing years of work and cumulative shifts. Lack of a consistent definition of night shift, which may skew results by resulting in a certain degree of misclassification. Many studies focused solely on a certain profession, which may result in an inapplicability to the general population. Many studies used self-report questionnaires, and the participants might not have reported truthfully. An improvement could be self-report questionnaires complemented by an assessment by experts which could improve the validity of shift work exposure assessment. This analysis indicates that there is an association between night shift work and the risk of breast cancer. Further research with higher quality cohort studies is needed to confirm these results. In order to be able to reduce the number of cases of breast cancer worldwide, risk factors such as night shift work need to be properly identified so that policies and changes in prevention can be made. Discussion Limitations Conclusion Night Shift Work and the Risk of Breast Cancer Jaskiran Kaur 1 & Carina Stumpf 1 Epidemiology 1 Faculty of Health Science at the University of Ottawa Research Question For females working at night, what does the evidence suggest regarding the association between night work and breast carcinoma? Figure 1. Increased risk of breast cancer has been associated with the amount of years spent working shift work at night. From BBC News at bbc.co.uk/news and by permission of Oxford University Press.bbc.co.uk/news Study Country and period of time under observation # of cases /controls or cohort size Exposure measure Level of Exposure Lie et al. (2011) Norway ( ) 699/895 Telephone interview a) 1–11 years of work in schedules that included night work. b)12 years of work in that included night work versus never having worked at night. Davis et al.USA ( )813/792 In-person interview Having worked night shifts versus never Menegaux et al. France ( ) 1232/131 7 In-person interview Having worked night shifts versus never Pesch et al. Germany ( ) 857/892 Telephone interview Having ever worked in night-shift work versus employed but not at night Hansen and Lassen Denmark ( ) 218/899 Questionnaire and telephone interview Having worked night shifts versus never Lie et al.(2013) Norway( ) 513/757 Telephone interview Having worked minimum of 6 consecutive night shifts versus never Grundy et al. Canada ( ) 1134/117 9 Questionnaire and telephone interview a) Having worked either 0-14, 15-29, or >30 years in postmenopausal women of night work b) Having worked either 0–9, 10–19 or ≥20 years in premenopausal women of night work versus never Hansen Denmark (1964– 1999) 7035 Interview-based Survey and Central Persons Registry Having worked night work versus employed but not in night work Tynes et al. Norway ( ) Follow-up( ) Age<50: 29/150 Age>50: 21/109 Job histories a) Age 50, low level of shift work with a score of 0–3.1 d) Age>50, high level of shift work with a score 3.1–20.7 versus never Schernham mer et al. (2006) USA( )115,022Questionnaires a) Having worked either >20, or 1-9 years in night work versus never Schernham mer et al. USA( ) Bi-annual questionnaires Having worked either 1–2, 3–5, 6–9, 10–14,15–19, 20-29, or 30 or more years in night shift work versus never Table 1: Study characteristics from the studies StudyOR or RR95% CI Lie et al. [6] Davis et al. [7] 2.3 (5.7+h per week of graveyard shif tversus no graveyard shifts) Manegaux et al. [8] Pesch et al. [9] Hansen et Lassen [10] Lie et al. [11] Grundy et al. [3] Hansen [5] 1.5 (all night work combined in trades with predominantly night work versus women in trades with less than 40% night work) Tynes et al. [12] Schernhammer et al. [4] 1.79 (20+ years of night work versus no night work) Schernhammer et al. [13]1.36 (30+ year of night work versus no night work) Table 2 : Results from the studies