Night shift work and breast cancer: a pooled analysis of population-based case–control studies with complete work history

Abstract

Night shift work has been suspected to increase breast cancer risk but epidemiological studies have been inconsistent due to heterogeneous assessment of exposure to night work. To overcome this limitation, we pooled data of five population-based case-control studies from Australia, Canada, France, Germany, and Spain into a single harmonized dataset using a common definition of night work including 6093 breast cancer cases and 6933 population controls. The odds ratio for breast cancer in women who ever worked at night for at least 3 h between midnight and 5 a.m. as compared to never night workers was 1.12 (95% CI 1.00-1.25). Among pre-menopausal women, this odds ratio was 1.26 [1.06-1.51], increasing to 1.36 [1.07-1.74] for night shifts ≥ 10 h, 1.80 [1.20-2.71] for work ≥ 3 nights/week, and 2.55 [1.03-6.30] for both duration of night work ≥ 10 years and exposure intensity ≥ 3 nights/week. Breast cancer risk in pre-menopausal women was higher in current or recent night workers (OR = 1.41 [1.06-1.88]) than in those who had stopped night work more than 2 years ago. Breast cancer in post-menopausal women was not associated with night work whatever the exposure metric. The increase in risk was restricted to ER+ tumors, particularly those who were both ER+ and HER2+ . These results support the hypothesis that night shift work increases the risk of breast cancer in pre-menopausal women, particularly those with high intensity and long duration of exposure. Risk difference between pre- and post-menopausal women deserves further scrutiny.

Prostate cancer risk decreases following cessation of night shift work

“…A recent large pooled analysis on breast cancer that examined also time-related variables showed that risk in premenopausal women was highest in those with current or recent (2 years) night shift work, decreased in women who had left night shift work 2-20 years earlier, and was lowest at 20 years or more since last night shift work. 1 A similar pattern was observed in a large cohort study in the UK 2 that overall did not find any association between breast cancer and night shift in older women, but reported a 10% increase the first 10 years after exposure while a decrease in risk was observed following that period. A relatively similar pattern although not entirely consistent was described for night shift and CLL.…”

“…Beatriz Pérez-Gómez Javier Burgos Inés Gómez-Acebo Javier Llorca Rosana Peiró Jose J. Jimenez-Moleón Juan Alguacil 1 Adjusted for age, education and centre. 2 Adjusted for age, education, centre and lifetime cumulative years of night work.…”

Rotating night shift work and colorectal cancer risk in the nurses’ health studies

“…Our exposure definition included rotating night shift work schedules with at least 3 nights per month and we assessed cumulative night shift work duration in years. However, recent evidence suggests that other exposure metrics such as shift work frequency (nights/month or number of consecutive nights) and length (hours/day) may also predict breast cancer risk in addition to cumulative lifetime duration (years) 30 . Our study lacked information on shift work frequency and length but these metrics should be considered in future studies.…”

Shift work and cardiovascular disease – do the new studies add to our knowledge?

“…The recent Million Women Study (8), which found no association between shift work and cancer, included subjects of very advanced age. Starting follow-up at an older age may have resulted in the inclusion of more participants less susceptible to the effects of night shift work (7,9). Furthermore, old age is associated with the accumulation of competing risk factors, diminishing the relative role of shift work compared to the other factors (10).…”

“…Some light is seen in the acknowledgement of the need for better exposure assessment of shift work. The new case-control studies of shift work and breast cancer used interviews to get more precise and relevant information on exposure to night and shift work (7). However, the use of electronic records of working hours would be most optimal.…”