Recent epidemiological evidence has associated night-shift work with adverse health effects, including increased cancer risk. While the reasons behind these negative outcomes are unclear, they may be due to alterations in the circadian clock, effects on melatonin secretion and the neuroimmune system, development of inflammation, effects on tumor suppressors, as well as possible impact on telomere length and engaging in unhealthy habits. (e.g., overeating) or experiencing psychic distress. However, evidence on the effect of night-shift work on breast cancer (BC) risk has been conflicting.

An updated meta-analysis was conducted of prospective cohort studies focusing on BC incidence and all-cause mortality. A literature search was conducted of MEDLINE via the Cochrane Library, Embase, ProQuest, and PubMed from inception through August 2020 of clinical prospective cohort studies, which examined night-shift work exposure, BC incidence, and all-cause mortality.

Studies that provided information on relative risk (RR) and 95% confidence interval for mortality of cardiovascular, cancer-related, or all-cause death were included. Case-control, cross-sectional, or retrospective studies, studies involving patients with recurrent BC, and those with exposure to non–work-related night-shift or involuntary night work, were excluded.

Dual meta-analyses were performed for the two outcomes. The endpoints for these studies were first diagnosis of BC, death attributed to specific cancer, or all-cause death.

The investigators identified 31 prospective cohort studies, which included 9,357,272 participants that met initial inclusion criteria. Of these study subjects, there were 31,244 cases of BC, 12,728 cancer-related mortalities, 7,882 cardiovascular mortalities, and 30,807 all-cause mortalities.

An association between night-shift work and all-cause mortality. The summary RR of BC incidence with night-shift work exposure was significantly increased (RR 1.029, 95% CI: 1.003-1.055; P = .027). Risk was not increased in the subgroup with 0 to 10 years exposure to night-shift work (RR 1.015, 95% CI 0.985-1.045; P =.328) However, a >10-year exposure was associated with a statistically significantly increased RR (RR 1.086, 95% CIU 1.032-1.42; P = .001). A positive association was found between BC risk and rotating night-shift work (RR 1.053, 95% CI 1.018-1.090; P = .003) but not with fixed night-shift work (RR 0.974, 95% I 0.916-1.035; P = .392).

Among the studies that examined the association between night-shift work and all-cause mortality, there was no increase in BC-related mortality observed (RR 1.241, 95% CI 0.941-1.637), nor was an association between night-shift work and pancreatic cancer, ovarian cancer, and all cancer-related mortality observed. However, night-shift work was positively associated with an increased risk of cardiovascular mortality (RR = 1.031, 95% CI 1.006-1.057; P = .015).

The authors discuss research into possible protective therapies to help mitigate BC risk, such as the use of melatonin, orexin receptor agonists, and targeted gene therapy for those with the germline BRCA1 or BRCA2 mutations.

Since pharmacists often work rotating shifts, including night shifts, this study is both reassuring and concerning regarding the effect that night-shift work has on overall cancer and mortality risk.

The content contained in this article is for informational purposes only. The content is not intended to be a substitute for professional advice. Reliance on any information provided in this article is solely at your own risk.

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