New cancer risk from 24-hour society

With more than a million British women now working through the hours of darkness, researchers from Harvard University have established that regular night shifts increase the chance of developing the disease by as much as 50 per cent. The more night work they do the higher the risk, says the study, which concludes that exposure to artificial lighting could be to blame.

Approximately one in six UK employees undertakes some form of shift work at night, particularly in the manufacturing, transport and communications sectors.

The doctors at Harvard University and Brigham and Women's Hospital (attached to Harvard) analysed data on more than 10,000 women, including several hundred airline cabin staff, and found that those who worked at night were 1.5 times more likely to get cancer than daytime staff.

One theory is that artificial lighting at night leads to a greater production of the female hormone oestrogen. Previous research has suggested that sleeping with the light on could cause breast cancer.

The research, published in the European Journal of Cancer this week, adds to a growing body of research raising concerns about the long-term health effects of shift work, and frequent changes between day and night duty.

Many call centres are now staffed permanently, and large sections of the retail and entertainment industries are also open round the clock. Previous studies have shown that working at night also increases the risk of heart disease, depression and other forms of cancer. One study of mortality rates suggested it is more damaging than smoking 20 cigarettes a day.

The hormone melatonin could also be part of the explanation. It is normally released into the body at night, and, among other things, it regulates sleep patterns. Women with high levels of melatonin in the morning are at lower risk of breast cancer.

When normal melatonin cycles are interrupted in women - possibly through exposure to artificial light - the production of oestrogen from the ovaries is higher,

"We found a significant 48 per cent increase in the risk of breast cancer among shift workers,'' said the Harvard researchers. "Exposure to artificial light at night, when the production of melatonin is at its physiological height, sharply reduces levels of melatonin.

"It has been suggested that the decreased melatonin production due to exposure to light at night leads to a rise in the levels of reproductive hormones such as oestrogen, thereby inducing hormone-sensitive tumours in the breast. The fact that risks for both flight attendants and other night work occupations were essentially identical provides an argument against previous theories suggesting that the increased incidence of breast cancer in flight attendants is due to effects of increased radiation or electromagnetic exposure.

"Rather, the observed increased risk of breast cancer may be associated with engagement in night work and a related decrease in melatonin production.''

Last week separate research from the Harvard Medical School published in the International Journal of Cancer suggested that eating chips as a young child may increase the risk of cancer.