Does shift work increase the risk of breast cancer?

Who is most at risk from night shift work?

The risk appears highest for women who work night shifts for many years and for older women. A 2021 meta-analysis of 26 studies found that women who worked night shifts for less than 10 years had a 13% higher risk of breast cancer (relative risk 1.13) compared to non-night workers, but the increase for those working 10 or more years was smaller and not statistically significant [2]. In contrast, a 2022 Finnish study found that women aged 50 or older who had done shift work (with or without nights) for over 10 years had about double the risk of breast cancer (hazard ratio around 2.0) compared to day workers [1]. A 2023 Finnish twin study reported that women working night shifts had a 58% higher risk (hazard ratio 1.58) than day workers, with the highest risk among those born between 1950 and 1957 (more than double the risk) [5].

Healthcare workers may be a particularly vulnerable group. A 2025 meta-analysis of healthcare workers found that those with 20 or more years of night shift work had a 25% higher risk of breast cancer (relative risk 1.25), though this association weakened after adjusting for potential biases [3]. The same study found a 68% higher risk for 30+ years, but this was based on only three studies and was not statistically significant [3].

Why don't all studies agree?

The main reason for conflicting results is that different types of studies give different answers. When you look at case-control studies (which compare women with and without breast cancer and ask about past night work), they consistently show a higher risk — for example, a Polish case-control study found night workers had 2.6 times the odds of breast cancer [7]. However, cohort studies (which follow healthy women forward in time) generally find no significant link. A 2021 meta-analysis reported that cohort studies showed no association (relative risk 0.98), while case-control studies showed a 34% increase [6]. Cohort studies are considered more reliable because they avoid recall bias — women with cancer may remember and report night work differently than healthy women.

In fact, a 2024 study directly tested this recall bias and found that women with breast cancer recalled night shifts slightly more accurately than healthy women (86% vs. 81% sensitivity), and both groups had low specificity (around 83%), meaning they often incorrectly reported never working nights [4]. When the researchers corrected for this misclassification, the apparent increased risk dropped from 12% to just 5% and was no longer statistically significant [4]. This suggests that some of the reported risk may be due to how night work is remembered, not a true biological effect.

What should you do if you work night shifts?

The bottom line is that the evidence is not strong enough to say night shift work definitely causes breast cancer, but there is enough concern that the International Agency for Research on Cancer (IARC) classifies it as a "possible human carcinogen" [2]. For an individual woman, the absolute increase in risk is likely small — most studies find a 10-60% relative increase, but because breast cancer is fairly common (about 13% of women will develop it in their lifetime), even a small relative increase can translate into many cases at the population level. For example, one study estimated that if the link were real, night shift work could be responsible for a substantial number of breast cancers among healthcare workers [3].

If you are concerned, focus on things you can control: maintain a healthy weight, limit alcohol, exercise regularly, and get regular breast cancer screening. Some studies suggest that longer sleep duration (over 8 hours per night) might amplify the risk from night work [5], so try to get consistent, quality sleep even if your schedule is irregular. The evidence is not strong enough to recommend quitting night work solely to prevent breast cancer, but if you have other risk factors (like a strong family history), you may want to discuss your schedule with your doctor.

Sources used in this answer

Related research questions