- A large cohort study finds that night shift work is unrelated to breast cancer onset.
Why this matters
- This is among the largest and longest studies to date.
- Findings may prompt the International Agency for Research on Cancer to revise its determination that night shift work is a probable carcinogen.
- The UK Generations Study is a prospective cohort of women without breast cancer (n=102,869 recruited in 2003-2014) who were followed for up to 10 years.
- Funding: Breast Cancer Now; The Institute of Cancer Research.
- Median follow-up was 9.5 years.
- No increased risk for breast cancer was seen with night shift work within the last 10 years (vs no night shift work; HR, 1.00; 95% CI, 0.86-1.15).
- A significant trend was seen with average hours of night work per week (P=.035), but no increased risks were found for:
- Hours worked per night.
- Nights worked per week.
- Average hours worked per week.
- Cumulative years of employment.
- Cumulative hours.
- Time since cessation.
- Type of occupation.
- Age starting night shift work.
- Age starting in relation to first pregnancy.
- Receptor status of breast cancer (estrogen receptor-positive, progesterone receptor-positive, or human epidermal growth factor receptor 2-positive).
- Observational design.
- Findings may not apply to other ethnicities.