Roger Dobson, "New cancer risk from 24-hour society", Independent,
August 21, 2005,
Link: http://news.independent.co.uk/uk/health_medical/article307345.ece

The 24-hour economy is placing women at a hugely increased risk of
breast cancer, a major study warns today.

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.