Working the Night Shift Increases the Risk of Breast Cancer

Trying to sleep when the rest of the world is working, or working when the rest of the world is asleep is not an easy thing to do. A recent news story about an Edmonton woman who quit her job as an armed guard because her children didn’t like her working nights, highlights just one of the disadvantages of working at night. But for women, working the night shift may also lead to breast cancer.

French researchers conducted a major population study between 2005 and 2008 that examined the effect of night work on women’s health. They looked at the careers of 3,000 women, including the amount of time they spent working at night.

Research findings

Women who worked night shifts had a 30 percent greater risk of developing breast cancer than women who never worked nights. The risk increased for women who worked more than four years of night shifts, or who worked nights less than three times a week. These women had the greatest disturbance of their day and night circadian rhythms.

Women who worked night shifts before their first pregnancy also had an increased risk of breast cancer. Researchers speculated that in these women, their mammary cells had not completely differentiated before their first pregnancy, making them more vulnerable to breast cancer.

Why is  working at night a problem?

A 2010 study by the International Agency for Research on Cancer (IARC) deemed work that disturbed the circadian rhythm “probably carcinogenic.” Our circadian rhythm regulates sleep and wakefulness and controls numerous biological functions. The circadian rhythm is altered in people who work at night or have disrupted working hours.

Why night work may lead to cancer

While more research needs to be carried out, scientists speculate that there may be several links between working nights and breast cancer:

• nighttime exposure to light disrupts natural melatonin levels, and impairs melatonin’s anti-cancer properties
• disruption of the circadian rhythm affects the functioning of genes that control cell proliferation
• sleep disorders may weaken the immune system

More than 1.3 million cases of breast cancer are diagnosed globally each year. Over 11 percent of the women surveyed had worked nights at some point in their lives. Unfortunately, in our 24-hour, have-on-demand world, more women are working nights and performing shift work than ever before.