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Does shift-work cause cancer?

by Ed Yong | Analysis

18 December 2007

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London at night.

This article is more than 3 years old. For a more recent picture, read our latest article on night shifts and breast cancer

“Everything in moderation” has become something of a catchphrase for cancer prevention. We know that many things can increase the risk of cancer if you get too much of them. Red and processed meat falls into this category, as we’ve blogged about before. Salt fits here too, as does alcohol.

Now, some scientists are debating whether light should join this list. Some studies have found that people working on night-time shifts could have higher risks of cancer after many years because of their exposure to “light-at-night”.

And last month, the International Agency for Research into Cancer (IARC) – an organisation whose business it is to classify potential cancer risks – surprised many people by categorising shift-working as a “probable” cause of human cancer.

But how strong is the evidence? How could a bit of night-time illumination affect the risk of cancer?

The idea seems a bit far fetched at first. Light is, after all, natural and everywhere. But the problem is that light only turned up everywhen relatively recently in human history.

Electric lights illuminate our cities 24/7 and many people have jobs where light shines on their bodies at times when darkness would be expected. This extra exposure has the potential to mess up our internal body clocks.

The darkness hormone

Our bodies work on internal daily cycle. We get sleepy at night and are most alert during the day. And just as the sun rises and falls in the sky, the levels of important molecules in our cells rise and fall over twenty-hour hours.

One of the most important of these molecules is melatonin, the ‘hormone of darkness’. It’s produced by the brain during times of darkness, and its levels are highest in the middle of the night. Light, however, stops the brain from churning out melatonin – shift-workers have abnormally low levels of this hormone.

Melatonin’s main job is to keep our biological clocks ticking over regularly. But scientists are finding out that it also has a whole suite of other jobs – some of which seem to be able to protect against cancer.

Various research studies have demonstrated that melatonin could lower levels of oestrogen in the blood, increase the activity of the tumour suppressor gene p53, boost the immune system, mop up free radicals and block cells from dividing. Indeed, studies in animals and cultured cells have found that melatonin can even block the growth of cancer cells and tumours.

So low levels of melatonin provide a biologically plausible link between shift-working and cancer. But what do studies in actual people say?

Shift-working and cancer risk

So far, most of the research has focused on breast cancer and on the whole, studies have consistently found that shift-workers have a higher risk of this disease. Two years ago, an overview of 13 different studies concluded that shift-workers had 48 per cent higher risks of breast cancer compared to other women.

Some studies have also linked shift-working with bowel, prostate and womb cancers, but only one or two studies have been done for each cancer type.

On the face of it, these look like compelling results, especially when taken together with the melatonin hypothesis. But many of these studies suffered from a major flaw – they didn’t account for other causes of breast cancer, including alcohol, body weight, physical activity, social class, having children and so on.

These are known as ‘confounding factors’ and they could be the real reasons behind any reported links. For example, we know that breast cancer is more common in inactive women – so if shift-workers get less exercise than the general population, this could explain their higher risk.

To the future

So to get a definitive answer, future studies need to rule out these alternative explanations. And these questions, certainly, need answers:

  • So far, the breast cancer studies have mostly looked at very specific groups of people. Seven of them focused on flight attendants, while several others looked at nurses. How do these results apply to the wider population?
  • A couple of studies found that only women who worked on shifts for several years (20 or even 30) had a higher risk of cancer. If there is an actual risk, does it only show up after decades of shift working?
  • Shift-working may be listed as a “probable” cause of cancer, but what about other sources of light-at-night? What about bedside lamps, street lights or late-night telly?

Clearly, this is a topic that warrants further research. The existing studies and the melatonin hypothesis are compelling but several issues need to be clarified by carefully designed studies.

Until then, it’s too early to suggest that the ‘graveyard shift’ is anything but a figure of speech.