Night-Shift Work – a link to breast cancer in women?
It is well established that there can be undesirable consequences for those working shifts outside standard daytime hours, particularly those covering the night or with early morning starts. Over the past few years, evidence has been emerging that suggests that night shifts are bad for you. Typical symptoms include: fatigue, disturbed sleep, digestive problems and a greater risk of accidents at work. Some studies also show a higher risk of breast cancer in women who sleep for fewer hours at night.
Danish Government pays out for breast cancer in shift workers
In March 2009, the Danish government paid compensation to around forty women who had developed breast cancer after long spells of shift work involving working at night. This decision followed a ruling by Agency for Research on Cancer (IARC, which is part of the United Nations World Health Organisation) that night shifts probably increase the risk of developing cancer. Part of the function of the IARC is to study and rank cancer risks. Category One risks are known carcinogens, including asbestos, but night-working has been categorised as only one level below that, i.e. a probable cause of cancer. The IARC reached this conclusion after looking at a wide number of studies in humans and in animals. A report published in the Journal of the National Cancer Institute reported a 36% greater risk of breast cancer for women who had worked night shifts for more than 30 years, compared with women who had never worked nights.
A hormonal mechanism for breast cancer
When faced with a diagnosis of cancer, people want be able to identify a single factor in their history that they can identify as the single cause of the disease. Cancer, however, is a complicated disease and rather than thinking in terms of a single cause, its best considered as being the result of a combination of many factors, which all have a small, but significant, contribution. In the case of breast cancer these factors would include: the woman’s genes, her age, her weight, her race, how much she drinks, when she started (and stopped) her periods, how many children she has had, whether (and for how long) she took birth control pills or hormone replacement therapy, and how long she breast-fed.
In 2001, several epidemiological studies providing evidence linking breast cancer in women with prolonged periods of working at night were reported. Within these studies, a biologically plausible mechanism related to the suppression of melatonin was suggested as a potential explanation for these findings. According to this theory, this association arises from reductions in serum levels of melatonin that follow from nocturnal exposure to light. Melatonin is known to suppress tumour growth in experimental animal models, and reduced melatonin levels may increase ovarian oestrogen release. In the normal pattern of life, our eyes sense the reduction of natural light levels. This triggers the pineal gland to begin the secretion of the hormone melatonin and as the melatonin levels rise in our bodies we tend to get sleepy. In women, as the melatonin levels rise, the production of the female sex hormone oestrogen decreases. It is believed that the production of oestrogen stimulates the growth of breast tissue including some breast cancers. It may be considered that more light results in less melatonin and in more oestrogen, which means a greater risk of cancer.
Information from studies
Nurse study - In 1988, nearly 80,000 nurses who had no history of breast cancer were questioned about previous overnight shift work. The study participants were then followed for ten years. The resulting analysis was then adjusted for potential confounding factors and it was found that women who had worked rotating night shifts for 30 years or longer had significantly increased risk for developing breast cancer compared with those who had never worked night shifts (a relative risk of 1.36). Among nurses with less than 30 years of shift work, risk was slightly elevated but fell just short of significance (a relative risk of 1.08).
Second study - In a separate study, 813 women with breast cancer were compared with 793 age-matched controls. A history of overnight shift work during the previous 10 years was associated with significantly increased risk for breast cancer (an “odds ratio” of 1.6, after adjustment for other risk factors). Breast cancer risk was also found to be increased significantly among women who frequently did not sleep during the middle of the night for any reason (there was an “odds ratio” of 1.7 for the group with at least 2.6 nights per week of interrupted sleep).
Danish Study - In a population-based case-control study, the breast cancer risk among Danish women aged between 30 and 54 who worked predominantly at night was investigated. Individual employment histories were reconstructed back as far as 1964 for each of 7035 women with breast cancer along with individually matched controls from the records of a nationwide pension scheme with compulsory membership. The “odds ratio” for breast cancer among women who worked at night at least half of a year was 1.5 and there was a tendency to increasing odds ratio by increasing duration of night time employment.
Practical steps to reduce the risks of breast cancer - for shift workers
Shift workers can reduce their personal risk by eliminating some lifestyle risk factors, by:
• stopping the use of tobacco
• keeping alcohol intake moderate
• exercising regularly
• maintaining a healthy weight
• getting enough sleep on a regular basis
• make sure that their bedroom is completely dark when they sleep (both at night and during daytime, to make sure that melatonin secretion is not affected)
“Be Breast Aware” – there is a free leaflet available from the NHS in a range of languages. Breast aware encourages women to:
• Take care of your own well-being
• Know what is normal for you
• Know what to look and feel for
• Report any changes without delay
• Attend for breast screening if aged 50 for over
Practical steps to reduce the risks of breast cancer - for the employer
• Train and educate shift workers on what constitutes a healthy lifestyle and how to achieve it and maintain good quality sleep
• Design shift schedules to allow sufficient rest and provide the opportunity to obtain adequate sleep.
• Keep overtime at a healthy level
• Introduce health surveillance for night shift workers
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