Could "the graveyard shift" be more than a figure of speech? The Danish government certainly thinks so. It has just begun to award compensation to women who have developed breast cancer after years of working night shifts - such as flight attendants and nurses. So far about 40 women have received pay outs, although not all claims were successful - including women who had a family history of breast cancer.
The Danes acted after the International Agency for Research on Cancer, part of the World Health Organisation, placed shift work along with anabolic steroids, ultraviolet radiation and diesel engine exhaust fumes as a "possible human carcinogen" in 2007. One level up are Category One risks - "known carcinogens" such as asbestos. The agency's conclusion was based on evidence from a wide number of studies of both humans and animals.
Most studies have concentrated on linking breast cancer with what cancer researchers call the "light-at-night" effect on the body's hormones, and they have consistently found that shift-workers have a higher risk of developing breast cancer than women who work regular daytime hours - on average around a 48% higher risk.
Scientists believe that this is because of the effects of shift work on the body's production of the so-called "hormone of darkness", melatonin. If your night time is spent in artificial light (even dim artificial light) your brain stops churning out melatonin. This multitasking hormone keeps your biological clock ticking over, making sure that you are alert during the day and sleepy at night. It also seems to play an important role in cancer protection.
Melatonin lowers levels of the female hormone oestrogen in the blood - oestrogen is known to encourage the growth of certain cancers, notably breast and ovarian cancer. It could also block the growth of cancer cells and boost the body's immune system by killing cell-damaging "free radicals" (killing free radicals also happens to be why antioxidants are so prized) and block cells from dividing. Since the brain produces the most melotonin in the middle of the night when it is dark, night- shift workers - whose bodies are saturated by artificial light - have abnormally low levels.
So how worried should night-workers be? "I think we can say there is a big public health problem here," says Professor Andrew Watterson, an occupational health specialist at Stirling University. We are not just talking about breast cancer (a significant enough problem in its own right), he says, there is also evidence linking the night shift with a raft of other health problems, including prostate or womb cancer. Night shifts might also have detrimental reproductive effects: a Danish study of 40,000 women found that night workers had more chance of having lower-weight babies (and lighter babies have a higher chance of serious health issues, such as respiratory problems or disabilities). These women were also more likely to have pregnancies that lasted longer - raising the risk of stillbirth.
Other studies have suggested a link with bowel cancer and gastric problems - possibly because night workers tend not to eat so healthily and to exercise less. There is also, says Watterson, evidence of a link between shift work and non-Hodgkins lymphoma, possibly because the body's immune system lacks the positive effects of melatonin. "The Danish response to the breast cancer issue is very progressive," he concludes. "The British government should absolutely follow suit."
The government currently has no plans for compensating British women in such a way. And according to Cancer Research UK, any night-shift panic would be premature. "The breast cancer risk has not been conclusively shown," says Dr Kat Arney, senior science information officer at Cancer Research UK. This is because there are so many complicating factors when you try to study the effects of lifestyle on cancer risk. "At the moment we just don't know how other lifestyle factors, such as taking HRT, obesity, having fewer children or drinking alcohol, interact with shift work to increase a woman's risk of breast cancer," says Arney.
All these factors - rather than melatonin - could be the real reasons behind any apparent cancer links. For instance, explains Arney, "we know that breast cancer is more common in inactive women, so if shift workers get less exercise than the general population, this could explain their higher risk."
"We'll know more about the risks to British women from an upcoming Health and Safety Executive study, which is due to be completed by 2011," she says. As for bowel, prostate or womb cancers, only one or two studies have been done for each cancer type, so more research must be done before anyone can say for sure that there is a link. "More research would be good," says Watterson, "but it's obvious that there are major health problems associated with shift work."
If there does turn out to be a "light-at-night" effect, then tackling it could be a major health challenge. For instance, how many shifts per week could you get away with before your risk of related cancers became significant? Watterson says it is probably safe to assume that, "You don't have to be working every night to be at risk. It just has to be part of your shift pattern, something you do periodically, but regularly."
The current studies show that if there is a link then it takes a long time to kick in - you'd need to work regular night shifts over 20 or 30 years to significantly raise your risk of developing breast cancer. Cancer Research UK says that until there is more conclusive evidence, shift workers - like anybody else - should go to the doctor if they notice any abnormalities and keep up with their NHS screening appointments.
Watterson thinks a more organised public health response is called for. "We don't tend to identify the damage being done where shift working is prevalent and I think that's an error," he says. "The damage is there but we don't see it and we don't count it."
The night worker's tale:
'I was a grumpy, spotty hermit'
Working as a journalist at ITN meant regular two-week spells of night shifts. I'd get a sickening sense of dread for the fortnight leading up to them, knowing that I would become a sleep-deprived grumpy, spotty hermit.
At 10pm on night one, instead of my usual wind-down glass of wine, I'd take a wake-up shower and set off to work, where I would struggle to stay alert until 8am.
Night two should get better - but only if you've remembered before going to bed to unplug your landline, turn off your mobile phone and deactivate your doorbell - and your neighbours don't decide to indulge in any DIY.
The office is an eerie place at night. There's no banter and it's hard to stay motivated. And I did no excercise - because I was living every waking moment in darkness, venturing into the outside world for my usual run seemed an impossibility.
By the end of the fortnight, I'd have purple under-eye bags, stomach cramps (my digestive system would get very confused), and emotions so highly charged ("Nobody understands how tired I am!") that I was fit to explode.
Helen Croydon
