Shift Work and Breast Cancer

Light Pollution

Adverse Effects of Shift Work

Nearly 30% of employed Canadians are shift workers. These people work during the night or evening rather than during the day. They may work on rotating shifts that switch from day to evening or night and back again.12 Many industries rely on shift workers to provide ongoing and continuous service. Examples include health care, manufacturing, and transportation (Table 1).13 The number of shift workers in Canada has increased steadily, from 10387 workers in 1992 to 13139 workers in 2005 (Figure 1).

Shift workers experience a number of adverse effects, with lack of sleep being the most common.2 In addition, shift workers report higher levels of workplace injury, stress and anxiety, cardiovascular disease, hypertension, reproductive health problems, gastrointestinal disorders, and cancer.

The association between shift work and breast cancer has become the focus of much attention especially for flight attendants and nurses. Research involving both occupations finds that night shift work is associated with increased risk of breast cancer in women who have worked rotating shifts for 30 years or longer. One explanation for this association is the light-at-night (LAN) theory, which proposes that exposure to artificial light contributes to breast cancer by slowing melatonin production and disrupting circadian rhythms.

Increased Breast Cancer Risk
The incidence of breast cancer is 30% to 40% higher in flight attendants than in the general population, even after accounting for other differences such as diet and lifestyle.4 Initially, investigators speculated that the higher rate of breast cancer was partly due to increased exposure to cosmic radiation, which is increased at higher altitudes. Since then, little evidence has been found to support this association. More recently it has been suggested that the increase in breast cancer rates is the result of melatonin deficiency. Flight crew members are exposed to light at night regularly as they travel across time zones, which may explain their increased risk of breast cancer. Studies of flight attendants in both the United States and Europe provide support for this association.567

In the Nurses’ Health Study,8 women who worked night shifts for longer than 30 years had moderate increases in breast cancer (Figure 2).9 This evidence is taken seriously by many because of the study’s strengths, including the prospective design, the large number of women followed (240 000), the long study period, and the fact that the researchers collected and controlled for other factors that influence breast cancer risk (e.g. age, family history of breast cancer, and socioeconomic status).

While most epidemiological studies of light at night have focused on groups of women with elevated rates of breast cancer, some studies have focused on women with lower rates of breast cancer. Most notably, women with visual impairments have been found to have lower rates of breast cancer than women without impairments.10 One possible explanation for the lower rate of breast cancer is that visually impaired women have higher levels of melatonin than women in the general population because their melatonin production is not suppressed by exposure to light.