Night shift work may increase risk of breast cancer

Increased risk of breast cancer is associated with frequent, long-term night shift work.

The risk is higher among self-described “morning people” who work for frequent, prolonged periods on the night shift.

Women assigned to frequent, long-term night shift schedules have about a 40% increased risk of breast cancer compared with those whose work schedules do not disrupt normal circadian rhythms. The risk seems to increase with increasing number of night shifts worked per week and with cumulative night shifts worked over time.

Published online May 29 in the Journal ofOccupational and Environmental Medicine, the nationwide case-control study nested within a cohort of over 18,000 women in the Danish army born between 1929 and 1968. Using a detailed 28-page questionnaire, Danish researchers compared the responses of 141 women with documented breast cancer to those of 551 age-matched healthy controls. The questionnaire covered work habits, whether the women considered themselves to be morning or night people, contraceptive and/or hormone replacement use, number of children born, menopausal status, and sunbathing habits.

Women who worked at least 3 nights per week for at least 6 years or more had at least double the risk of breast cancer compared with those who worked less often at night. However, women who said they preferred mornings and worked mostly at night had 4 times greater risk of developing breast cancer than those who never worked at night. In the authors’ opinion, this warrants further exploration because it may be that women with a morning preference are less able to tolerate night work.

No increased risk was noted for women who worked fewer than 3 nights per week.

Exactly how night work puts women at greater risk of breast cancer is unclear, although fewer hours of quality sleep and decreased levels of melatonin are possible factors.

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