There is another theory that tries to explain the high incidence of breast and colo-rectal cancers in shift workers, however.
Melatonin is called "the Dracula hormone" because it always comes out at night. But its production can be severely reduced by bright artificial light. The effects of melatonin on health are not properly understood, but a number of scientists, particularly in America, are connecting low levels of melatonin with high levels of certain cancers in nightshift workers. One study presented to the American Association for Cancer Research found that melatonin can slow tumor growth by up to 70 percent in mice infected with human breast cancer cells.
When the mice were subjected to constant light, cancer growth rocketed.
Some have taken it further. George Brainard, a neurologist at Thomas Jefferson University in Pennsylvania, has recommended that we all exercise a "prudent avoidance" of light at night to ensure normal levels of melatonin whether we work night shifts or not.
Brainard's research has shown that the human body clock can be affected by light of short wavelength, which is more prevalent in artificial light used at night. It also showed that melatonin production was reduced by just this sort of short wavelength light.
Until recently, it was believed that only daylight was strong enough to influence our internal systems.
On the back of this, another researcher has advised parents not to let children sleep with the light on because of a potential - though unproven - connection between artificial light at night and childhood leukemia. The incidence of leukemia in children under five rose by 50 percent in the second half of the 20th century, leading some scientists to point the finger at our increased reliance on artificial light. "I would not myself use a nightlight in a kid's bedroom unless there is a reason for it for safety," said Richard Stevens, a cancer epidemiologist at the University of Connecticut, after a conference on childhood leukemia in London last autumn.
"There is interesting evidence about melatonin having properties that would lead to reduction in cancer risks, so the possibility that this might be related to childhood leukemia is important."
Foster, on the other hand, believes that many of these claims are "overselling" the evidence of melatonin's anti-cancer properties.
"It's true that melatonin is suppressed by light, but in reality we don't really know what effects it has on health. Its anti-cancer properties are a long way from being established as fact," he says.
"My opinion is that the suppression of the immune system is much more likely to explain cancer rates in shift workers."
Embrace the Dark