Night Work and Breast Cancer: A Review of Recent Research

Introduction to the Study

Researchers in Korea have recently conducted a comprehensive review of existing studies to explore the connection between night work and breast cancer. The International Agency for Research on Cancer (IARC) first categorized shift work that disrupts the circadian rhythm as “probably carcinogenic to humans” in 2007. This classification has raised significant concerns about work-related breast cancer, particularly given the high prevalence of night shifts and the increasing incidence of breast cancer. The International Labor Organization has reported that over 2.5 million individuals are engaged in night-shift work.

Breast Cancer Statistics in Korea

In Korea, breast cancer ranks as the second most common cancer and has seen rapid increases, particularly noted in 2013. Current established risk factors for breast cancer, including genetics and lifestyle, account for only about 50% of cases, prompting further investigation into other potential contributors.

Study Findings Published in Annals of Occupational and Environmental Medicine

The South Korean researchers published their findings in the Annals of Occupational and Environmental Medicine. Their study synthesized evidence from 21 original articles, which included both cohort and case-control studies, as well as five meta-analyses.

Link Between Night Work and Breast Cancer

The review indicated a potential association between night work and breast cancer. Specifically, four out of seven cohort studies demonstrated an increased risk of breast cancer among female night workers. Additionally, six of the 14 case-control studies reported a significant rise in cancer risk among groups with high exposure to night work. Notably, a meta-analysis conducted in 2005 confirmed an increased breast cancer risk for night shift workers, with four subsequent meta-analyses in 2013 yielding similar conclusions. However, it is important to note that no clear dose-response relationship was identified for those exposed to less than 20 years of night work.

Biological Mechanisms

The biological basis for the association between night work and breast cancer is supported by evidence from both animal studies and experimental data. Research indicates that melatonin, a hormone produced predominantly during the night, plays a crucial role in providing an anti-cancer signal to cells. Night shift workers often exhibit lower levels of melatonin due to their exposure to light during nocturnal hours. Furthermore, animal studies have shown accelerated tumor growth in conditions of constant light exposure, suggesting that diminished melatonin production may be a key mechanism linking night shift work to increased breast cancer risk.

Varying Definitions of Night Work

Challenges in understanding the relationship between night work and breast cancer arise from differing definitions of “night work” across countries. For instance, Korea defines night work as occurring between 22:00 and 06:00, while in the UK and France, it is defined as work from 00:00 to 05:00, and in Belgium, from 20:00 to 06:00. The discrepancies in the assessment of night work exposure across studies further complicate the ability to draw definitive conclusions. Future research should aim for greater consistency in these definitions and assessments.

Conclusion and Recommendations

Overall, evidence suggests a noteworthy association between night work and breast cancer risk, warranting recognition of night work as an occupational disease for patients with significant exposure. Outside of Denmark, few countries provide compensation for work-related breast cancer. There may be merit in establishing regulations concerning occupational factors, including frequency of night shifts, work schedules, total working hours, and exposure durations, to mitigate breast cancer risk among night workers.

Reference

Lee, H., Lee, J., Jang, T., Kim, I., Park, J., & Song, J. (2018). The relationship between night work and breast cancer. Annals of Occupational and Environmental Medicine, 30(1). doi:10.1186/s40557-018-0221-4