Is there a link between night shift work and breast cancer?

What the strongest studies actually find

The most reliable studies—large prospective cohort studies that follow people forward in time—show only a tiny or nonexistent link between night shift work and breast cancer. A 2021 meta-analysis of 31 prospective cohort studies, involving over 9 million participants, found that night shift work increased breast cancer risk by just 2.9% overall [5]. Another 2021 meta-analysis of 32 studies reported no significant association in cohort studies, with a relative risk of 0.98 (meaning no increased risk) [4]. These are the highest-quality study designs, and they consistently show that if there is any effect, it is very small.

Even the largest single prospective study here—the Sister Study, which followed over 48,000 women for an average of 9 years—found little to no increase in risk. Ever working rotating night shifts gave a hazard ratio of 1.08, meaning an 8% increase, but this was not statistically significant [7]. The study concluded that if night shift work does increase breast cancer risk, the increase is small.

Why some studies show a bigger risk

The reason some studies report a stronger link is that they use weaker study designs or have methodological flaws. Case-control studies, which ask people with and without breast cancer to remember their past night shift work, tend to show larger effects. For instance, a 2021 meta-analysis found that case-control studies showed a 34% increased risk, while cohort studies showed none [4]. This is likely because people with breast cancer may recall their night shift work more accurately (or differently) than healthy controls—a bias called recall bias.

A 2024 study directly tested this recall bias by comparing self-reported night shift work against objective payroll records. It found that women with breast cancer had slightly better recall of night shifts than controls, and that this misclassification caused a small overestimation of risk—correcting for it reduced the odds ratio from 1.12 to 1.05, which was no longer statistically significant [3]. This suggests that some of the reported link may be due to how studies measure exposure, not a real effect.

Does working nights for many years matter?

The evidence for long-term night shift work (10, 20, or 30+ years) is inconsistent. Some meta-analyses find a small increased risk for very long durations—for example, a 2025 review of healthcare workers found a 25% increased risk for 20+ years of night work, but this lost statistical significance when adjusted for publication bias [1]. Another 2021 meta-analysis found no significant association for over 20 years of night work [4]. A 2021 meta-analysis of prospective studies found an 8.6% increased risk for more than 10 years of exposure [5], but this is still a small effect.

Interestingly, some studies found a higher risk for short-term night work (less than 5 years) than for long-term work [7][2]. This counterintuitive finding suggests that other factors—like the type of night work (rotating vs. fixed), the population studied (e.g., flight attendants vs. healthcare workers), or unmeasured confounders—may be driving the association. For example, a 2021 meta-analysis found that flight attendants with long overnight flights had elevated risk, but noted that unmeasured confounders limited these results [2].

About These Sources

This answer is built on 7 peer-reviewed studies — published from 2021 to 2025, 2 from 2024 or later, 2 in Q1 journals, collectively cited 151 times — selected as the most relevant from 12 studies that passed quality screening, drawn from 42 papers retrieved from a database of over 500 million.

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