According to several studies evidence is mounting indicating that sleep interruption may significantly increase the risk of women developing breast cancer and other cancers, particularly those who work night shifts.1-4
Breast cancer is a common malignancy in the United States, with over 200,000 cases diagnosed each year. In response to the high incidence of this disease, researchers have focused efforts on screening and prevention strategies. Now, results from three separate clinical studies provide data indicating that women who work graveyard (night) shifts have a higher rate of breast cancer compared with women who work during the day.
Irregular working hours, including night and shift work, have long been considered a health risk. Irregular work schedules are becoming increasingly more common in industrialized societies. Approximately 26% of men and 18% of women in the United States labor force work variable shift schedules. Approximately 20% of the female labor force in Denmark works at night. Researchers from the Danish Cancer Society conducted a case-control study to investigate the relationship between night work and breast cancer.
The researchers identified 7,035 women with primary breast cancer from the database of the Danish Cancer Registry. They obtained information about past employment by record linkage with the nationwide pension fund. They then matched a control to each subject by random selection from the Central Population Registry database. The controls were alive and without cancer.
The researchers used the employment histories to determine whether women worked predominantly at night. Women were considered to work predominantly at night if they were employed for at least half a year in one or more of the trades in which at least 60% of females typically had nighttime schedules.
After adjustment for childbearing status, alcohol consumption and socioeconomic status, the results indicated that women who worked for at least half a year in any of the trades with predominantly night work were 1.5 times more likely to develop breast cancer. This risk was further increased as the duration of night work increased. The risk of breast cancer among women who worked at night for over 6 years was 1.7 times higher than the risk for daytime workers.
The researchers concluded that women who work predominantly at night are at an increased risk of developing primary breast cancer. While more research is needed to further define the relationship between breast cancer and night work, women who work at night or who are concerned about the risk of breast cancer may wish to speak with their physicians about appropriate screening for this disease, as the best treatment for cancer is preventing its occurrence.
Another study conducted by researchers at the Fred Hutchinson Cancer Research Center, involved over 800 women who were diagnosed with breast cancer between 1992 and 1995. These women were interviewed in person to gather information on risk factor exposure in their lives including history of family cancer, reproductive history, alcohol consumption, cigarette smoking, sleeping habits, etc. Their answers were compared to women of the same age who did not have breast cancer.
Results from the first study indicated that graveyard shift work increased the risk of breast cancer by up to 60%. There was an increased risk with increasing years of working at night and with increasing hours per night shift. Among women who did not sleep between 1 and 2 a.m., there was a 14% increased risk of breast cancer for every night worked per week. In addition, there was an indication of an increased risk of among women with the brightest bedrooms.
Researchers at the Brigham and Women’s Hospital and Harvard Medical School also reported results from a study in over 80,000 women that indicated an 8% increased risk in breast cancer in women who worked the night shift for up to 29 years and a 36% increase in breast cancer in women who worked the night shift for more than 30 years.
It is speculated that the hormone melatonin is somehow involved in the development of breast cancer in patients whose sleep is disturbed. Melatonin is released in the absence of light and is thought to be involved in the suppression of tumors through several biological processes. Melatonin is believed to directly suppress proliferation of cells, stimulate the immune system, scavenge free radicals, and modulate the expression of an important gene involved in cancer called the p53 gene. Laboratory studies have also indicated that impaired pineal secretion of melatonin is associated with an increased release of estrogen by the ovaries. Sleeping during the day or being exposed to light at night decreases the synthesis of and release of melatonin.
These results are important as they suggest an indirect association between melatonin and breast cancer. Women who work graveyard shifts should be especially diligent in screening procedures for breast cancer in order to catch the disease early when it is most curable.
References:
- Journal of the National Cancer Institute, Vol 93, No 20, pp 1513-1515, 2001
- Epidemiology, Vol. 12, No. 1, pp. 74-77, 2001
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8123502/
