10 January 2008

One in five European workers could be at increased risk of cancer because they work shifts, an international panel of experts has found.

According to the International Agency on Cancer (IARC), shiftwork that disrupts circadian rhythms - the so-called body clock - is "probably carcinogenic to humans".

Experts from 10 countries gathered at IARC in October 2007 to examine the scientific evidence on cancer and shiftwork - the first time the group has discussed the issue - and decided that shiftwork probably causes cancer in humans.

Among the evidence IARC evaluated were eight epidemiological studies on cancer in shiftworkers from several countries. Six out of these eight studies found a "modestly increased risk of breast cancer" in long-term shiftworkers.

The evidence from animal studies was stronger. In the 20-plus animal studies IARC looked at, most found major increases in cancer in rodents that were exposed either to constant light, dim light at night or simulated jet lag.

More than 20% of workers in Europe and North America - particularly those in the healthcare, hospitality, transport and communications sectors - work shifts. As a result, IARC called for more research on the health risks they face.

"Because there is credible evidence linking these occupations with increased risks of cancer, it is important that further studies be conducted to better identify what it is about such occupations that may increase the risk of cancer so that preventive measures can be implemented to avoid such risks," said IARC's director, Dr Peter Boyle.

Though IARC found more evidence of a link between cancer and shiftwork in animal studies than in humans, the biological mechanisms behind the relationship are highly plausible, IARC says: "Experimental studies show that reducing melatonin levels at night increases the incidence or growth of tumours ... These results may be explained by the disruption of circadian rhythms that is caused by exposure to light at night. This can alter sleep-activity patterns, suppress melatonin production, and disregulate genes involved in tumour development."

A summary of IARC's conclusions were published in the journal Lancet Oncology and IARC will publish its full findings in 2008. The full article is available at www.thelancet.com/journals/lanonc/article/PIIS147020450770373X/fulltext

 

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