Shift work and cancer
Some studies have raised concerns that working in shifts or being exposed to light at night could increase the risk of cancer, particularly breast cancer. Scientists are trying to work out if this is actually true and for now, the evidence is still uncertain.
In 2007, the International Agency for Research into Cancer (IARC) concluded that working night-shifts might increase the risk of cancer. IARC classified shift-working in its second highest category of risk (out of a total of four). This means that there is some evidence that shift-working could affect the risk of cancer, but that more research is needed to say for sure.
What does the evidence say?
There have only been a few studies looking at cancer rates in people who work night-shifts compared to those who do not. Most of these have focused on breast cancer. The majority of these studies have found that people who work night shifts for a long time are slightly more likely to develop breast cancer than those who do not.
But many of these studies had important flaws.
- They did not account for other things that can affect a person’s breast cancer risk. These include their body weight, how much alcohol they drink, how many children they have, their social status, whether they use hormone replacement therapy, and so on. Studies that did adjust for these other risks found a weaker link between shift-working and breast cancer.
- The studies also used different definitions of what “shift-work” actually is.
- Many of them only focused on a single group of workers, such as nurses or flight attendants. So it is hard to say whether the results apply to all people who work night shifts.
The evidence is stronger in animal studies. It seems that disrupting an animal’s body clock can increase the risk of cancer. But the picture is much less clear for shift work and breast cancer in people.
How could shift work lead to breast cancer?
Our bodies work on an internal daily cycle, known as our “body clock”. Shift-working can disrupt this body clock, which might affect our health.
Melatonin is a hormone produced by the brain’s pineal gland and is linked to our body clock. It is produced during hours of darkness, and levels peak in the middle of the night. Light, however, stops melatonin production and shift-workers have abnormally low levels of this hormone.
This is important because melatonin could protect against cancer in many ways. It could lower levels of oestrogen in the blood, increase the activity of genes that prevent cancer, boost the immune system, and block the growth of cancer cells.
So it is possible that shift-working could affect the risk of cancer by lowering levels of melatonin. However, other things like drinking alcohol at night could also affect our melatonin levels. This is why it is important to consider other causes of breast cancer when looking at the health of shift-workers.
Shift work and other cancers
Some studies have also linked shift-working with bowel, prostate and womb cancers, but only a handful of studies have been done for each cancer type. There is not enough evidence yet to suggest a direct link between shift work and any of these types of cancer.
However, this is an important area of research, especially since around 3.6 million people in the UK do shift work.
Although the evidence is not yet conclusive, the Danish government has compensated female shift workers who have developed breast cancer. Only women who have worked shifts for more than 20 years and who are otherwise at low risk for the disease were awarded compensation.
In the UK, the Health and Safety Executive is in charge of health and safety at work. They are funding further research into how shift work affects our health. The results of this study are expected by the end of 2015 and will hopefully shed more light on this topic. The report will help UK policy-makers decide whether any changes to working practice are required.
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