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.

长期上夜班的女性容易患乳腺癌、子宫癌，男性易患前列腺癌，许多疾病都与上夜班有关。丹麦政府已经开始给由于长年上夜班患上乳腺癌的妇女发放经济补贴。英国政府还没有类似的计划。

夜班被比喻为“墓地”轮班，应该不仅仅只是一个修辞比喻而已吧？丹麦政府就是这么认为的。丹麦政府现在开始给那些因为长年上夜班而患上乳腺癌的妇女——比如空姐和护士——发放补偿金。已经有40个妇女收到了补偿金，不过并不是所有的人去认领都能成功——不成功的人包括那些本来就有乳癌家族史的妇女。

2007年，丹麦政府根据世界卫生组织下属机构癌症研究署的研究结果，认为夜班与接触到合成类固醇、紫外线辐射以及柴油发动机所排出的废气这些“可能的致癌物质”一样，都可能导致患上癌症。而采取此次行动。（比可能致癌物）排名更高的是一类致癌物——“已知的致癌物”，例如石棉。这个机构是根据在大范围的人和动物研究后得出证据的基础上得出该结论的。

癌症研究人员将上夜班对人体荷尔蒙的影响称为“夜晚的灯光”，很多研究都集中在上夜班和乳腺癌之间的联系上。而且他们都发现了上夜班的妇女比上白班的妇女得乳癌的机率要高得多——平均要高出48%。

科学家认为上夜班对人体的影响是由于它会干扰人体制造所谓的“黑暗荷尔蒙”——抗黑变激素。如果你整个晚上都使用人造光（即使是很微弱的人造光）你的脑袋就会停止生产抗黑变激素。而这种多任务荷尔蒙可以让你的生物钟持续运转，保持你在白天的时候清醒，晚上的时候睡觉。它还在预防癌症方面有不可小觑的功能。

抗黑变激素会降低女性血液里的雌激素——雌激素是公认的刺激某些癌症生长的激素，比较显著的例子是乳腺癌和子宫癌。抗黑变激素还会阻止癌细胞的增加，并且通过杀死破坏细胞的“自由基”（杀死自由基也是为什么抗氧化剂能受到如此巨大欢迎的原因）来提高人体的免疫系统能力，它还能防止癌细胞。大脑在晚间会产生最多的抗黑变激素，而当黑夜加深，上夜班的工人在人造灯的照射下时，抗黑变激素的水平就会不同寻常的低。

所以夜班工人应该怎样担心？“我想我们可以说这是一个比较大的公共健康问题，”安德鲁沃特森教授说，他是斯特林大学的专业健康专家。我们并不仅仅只谈乳腺癌（当然这个问题非常严重），他说，还有一些其他的健康问题与夜班工作有关系，包括前列腺癌或者子宫癌。夜班还可能对生育造成影响：丹麦通过对4万个妇女的调查后发现，夜班工人比白班工人生的小孩的体重普遍要轻（而比较轻的小孩比较容易患上严重的健康问题，比如呼吸系统问题或者残疾）。这些妇女的怀孕期还可以能持续较长的时间——这就增加了死胎的风险。

还有一些研究表明夜班与胃癌和肠癌也有关系——可能因为夜班工人吃得比较不健康而且锻炼的也不够造成的。沃特森说还有一些证据表明夜班与非霍其金淋巴瘤有关系，这个可能是因为身体的免疫系统缺少抗黑变激素的刺激造成的。“丹麦对于乳腺癌问题的回应是站在世界前列的，”他总结到。“英国政府绝对也需要这么做。”

英国政府现在还没有计划像丹麦政府一样发放补偿金。根据英国癌症研究院所的看法，任何对于夜班的恐慌都还太早。“夜班工作导致妇女患乳腺癌的风险并没有完全显示出来，”博士Kat Arney说，她是英国癌症研究院的高级科学信息主任。这就是为什么当你研究生活方式对癌症风险的影响时有那么多复杂的因素需要考虑。“现在我们并不知道其他生活方式的影响，比如做HRT，肥胖，少要小孩或者喝酒，这些与夜班一起都会增加女性患乳腺癌的风险。”Arney说。

所有这些因素——比起抗黑变激素——更可能是造成癌症的真实原因。例如，Arney解释道，“我们知道乳腺癌在缺少活动的妇女身上是比较普遍的，因此如果夜班工人比起普通人来说，锻炼的比较少，这也可以用来解释为什么夜班工人患上乳腺癌的几率会比较大。”

“我们将会从即将出版的健康和安全指南研究里知道英国妇女面临的更多的风险，这个研究结论将在2011年出版。”她说。但对于肠癌、前列腺癌或者子宫癌患者来说，每一种类型的癌症都只有一到两个研究组在进行研究，所以只有在做了更多的调查以后我们才能明确的知道他们之间的联系。“有更多的调查最好，”沃特森说，“不过很明显夜班与一些主要的健康问题是有关联的。”

如果研究的结果真的证实是由于“夜晚的灯光”的影响，那么如何处理就会成为对健康问题的主要挑战。例如，在你与患癌症有关联的风险变得巨大之前，每周要减少多少轮班？沃特森说可以明确的说“你不必每天都冒着风险上夜班。这只是你轮班的一种形式，你可以周期性去做，不过要有规律。”

现今的研究表明如果两者之间有关联也需要一段较长的时间来证实——你需要有规律的上20年到30年的夜班以后才会大量增加你患上乳腺癌的可能。英国癌症研究所说直到有更多的证据以前，夜班工人——跟其他人一样——在发现任何异常后都应该去找医生，配合他们的国民健康保险制度上所写的条款进行治疗。

沃特森认为需要组织更多的公共健康机构。“我们不是要明确夜班已经造成的普遍伤害是什么，而且我认为这种想法是错的，”他说。“伤害已经存在，我们只是没有看到，没有去思考而已。”

夜班工人的故事：

“我是一个性情乖戾的，情绪不稳的隐士”

在ITN当记者意味着有规律的为期两周的夜班轮换。十四天的夜班后我发现感到胆战心惊的害怕，我很清楚的知道自己会变成一个睡眠不足的，性情乖戾情绪不稳的隐士。

在一个晚上的10点，我没有像平时一样喝一杯放松酒，而是洗了一个让自己能清醒起来的澡，然后出去工作，在那里，我要一直保持清醒直到第二天早上8点。

两个晚上以后情况就会好点——不过你要记得在上床睡觉之前关掉通讯工具，关掉手机还有拔掉门铃——还有你的邻居不会自己闯进来。

晚上的办公室是个比较怪异的地方。没有玩笑，而且很难保持积极的心态。而且我没有做任何锻炼——因为我醒着的时候都在黑暗之中，不太可能冒险出去外面跑下步。

14天过完后，我都会眼圈发黑，胃部抽筋（我的消化系统可能变得混乱了），而且我的情绪也会变得很情绪化（“没有人知道我有多累！”）需要发泄。