A team of 44 scientists in 10 countries commissioned by the World Health Organization’s International Agency on Cancer Research(IARC) issued a report on Friday, according to which shift workers have a higher risk of cancer than the general population.

Researchers discovered that shift work disturbs the body’s internal clock and that seems to have cancer-causing effects. The body’s internal clock is responsible for regulating circadian rhythm, a complex system that signals cells to produce various hormones at various times.

"Shiftwork that involves circadian rhythm disruption is probably carcinogenic to humans," write Kurt Straif, MD and colleagues.

The scientists reviewed studies in which animals were exposed to light at night, disrupting their body clocks (Circadian rhythm). The same thing happens to people exposed to the same conditions.

The scientists concluded that shift work might raise cancer risk by suppressing producing of melatonin, a chemical involved in the circadian rhythm.

Melatonin is known to act as an antioxidant protecting DNA from the type of damage that leads to cancer and heart disease.

Melatonin can be taken as a supplement, but scientists do not recommend its long-term use, since it could ruin the body’s ability to produce it naturally.

People who lack sleep are also predisposed to cancer risk. Not getting enough sleep makes your immune system vulnerable to attack and less able to fight off potentially cancerous cells.

"Timing is very important," said Mark Rea, director of the Light Research Center at Rensselaer Polytechnic Institute in New York, who is not connected with the IARC analysis.

If this theory proves to be correct, millions of people worldwide could be affected.

“Nearly 20% of the working population in Europe and North America is engaged in shiftwork. Shiftwork is most prevalent in the health care, industrial, transportation, communications and hospitality sectors. Among the many different patterns of shiftwork, those that include nightwork are most disruptive to the circadian system," said the report.

However, Vincent Cogliano of IARC said that this was the first time when IARC had examined shift work as a possible cause for cancer. He added that the result was not yet clear enough for anyone to take action and that more study is needed to prove its certainty.

"Then we would like the national health agencies to look at it and see what kind of action is appropriate," he said in a telephone interview.

Another research, led in 2001 by a team of researchers at the Fred Hutchinson Cancer Research Center in Seattle, discovered that women working night shifts might have a 60 percent greater risk of breast cancer.

Other studies revealed that firefighters also face a higher risk of cancer and heart disease due to exposure to smoke, chemicals, dust and work shifts.

Some research also suggests that men working at night may have a higher rate of prostate cancer.

The report will appear in the December Issue of The Lancet Oncology medical journal.