2 Studies Back View That Night Work Increases Risk of Breast Cancer

Two new studies support the view that working at night may increase a woman's risk of developing breast cancer, according to a report published in The Lancet Oncology.

Researchers at the Fred Hutchinson Cancer Research Center in Seattle studied 813 night workers and 793 matched controls. They found that the night workers had a 60 percent increase in relative risk of breast cancer, rising to 100 percent in women who worked at least 3 nights per week for more than 4.5 years during the previous decade.

Researchers have found that bright light suppresses the normal melatonin production, which can increase the release of estrogen, stimulating the turnover of breast epithelial stem cells and increasing the risk of malignancy.

Women who did not sleep when melatonin levels would have been at their highest had a 14 percent increase in risk for each night per week without sleep, according to the study.

"Turning on the light briefly doesn't increase the risk, but prolonged wakefulness in relatively bright light seems to be an important factor" said Scott Davis, lead researcher.

A second study from Harvard Medical School showed a more moderate level of risk for night shift workers with rotating shifts. Researchers studied 78,562 nurses, 2,411 of whom had primary breast cancer.

Nurses who had been on rotating shifts for one to 29 years had an 8 percent increase in relative risk compared to nurses who worked day shifts.

For nurses who worked at night for more than 30 years, the increase was 36 percent.

Researchers have found that bright light suppresses the normal melatonin production, which can increase the release of estrogen, stimulating the turnover of breast epithelial stem cells and increasing the risk of malignancy.

Other Sources: Lancet Oncology