What you may have heard
Frequent night shift work may be associated with a higher risk of breast cancer in women.
What the science tells us
Essential to many industries, night shift work can take a toll on the body by disrupting its internal clock and interfering with biological processes. Circadian disruption occurs when sleep patterns or exposure to light and darkness are significantly disturbed, causing the body’s rhythms to fall out of sync (CDC). This disruption can suppress melatonin production, a hormone that helps regulate sleep and may play a role in preventing tumor growth. Existing research suggests a link between long-term night shift work and breast cancer incidence.
Epidemiological Evidence
Findings from several studies suggest that long-term night shift work (20+ years) is associated with an increase in breast cancer risk. While some studies present greater effects than others, the overall evidence points to at least a modest elevation in risk among night shift workers.
A meta-analysis of observational studies found that night shift work raises breast cancer risk particularly in women who began shift work before menopause. The risk appears to increase with longer duration, higher frequency, and greater cumulative exposure (Hong et al.). Similarly, data from the Nurses’ Health Study I and II, two large prospective cohort studies, showed that long-term, rotating night shift work is linked to a higher risk of breast cancer, notably among women who completed shift work at younger ages (Wegrzyn et al.).
Additional research is needed to better understand the biological mechanisms by which night shift work influences breast cancer development and how this relationship varies across different populations.
Laboratory Evidence/Supporting Evidence
An experimental study in mice found that weekly light-dark changes, which simulate night shift work, may weaken the ability to stop tumor growth (Van Dycke et al.). This supports the idea that disruption of the natural circadian rhythm can promote breast cancer development.
IARC Carcinogen Classification:
“Probably carcinogenic to humans” (Group 2A)
How to reduce your risk
If you are a night shift worker:
- Limit long-term night shift work if possible, especially rotating shifts or frequent overnight hours over many years.
- Get routine physical examinations and cancer screenings.
- Prioritize sleep hygiene. Aim for consistent, high-quality sleep in a dark, quiet environment during the day.
- Maintain a healthy lifestyle through eating a balanced diet, exercising regularly, limiting alcohol, and avoiding smoking.
Bottom line
Night shift work is likely associated with an increased risk of breast cancer, especially with long-term or frequent exposure. Disrupting the body’s internal clock may interfere with hormone regulation and other biological processes related to cancer development. If you work night shifts, taking steps to support your health and reduce circadian disruption may help lower your risk.
Curious about how to improve your sleep? Watch this short video from Johns Hopkins University.