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.

長期上夜班的女性容易患乳腺癌、子宮癌，男性易患前列腺癌，許多疾病都與上夜班有關。丹麥政府已經開始給由于長年上夜班患上乳腺癌的婦女發放經濟補貼。英國政府還沒有類似的計劃。

夜班被比喻為“墓地”輪班，應該不僅僅只是一個修辭比喻而已吧？丹麥政府就是這么認為的。丹麥政府現在開始給那些因為長年上夜班而患上乳腺癌的婦女——比如空姐和護士——發放補償金。已經有40個婦女收到了補償金，不過并不是所有的人去認領都能成功——不成功的人包括那些本來就有乳癌家族史的婦女。

2007年，丹麥政府根據世界衛生組織下屬機構癌癥研究署的研究結果，認為夜班與接觸到合成類固醇、紫外線輻射以及柴油發動機所排出的廢氣這些“可能的致癌物質”一樣，都可能導致患上癌癥。而采取此次行動。（比可能致癌物）排名更高的是一類致癌物——“已知的致癌物”，例如石棉。這個機構是根據在大范圍的人和動物研究后得出證據的基礎上得出該結論的。

癌癥研究人員將上夜班對人體荷爾蒙的影響稱為“夜晚的燈光”，很多研究都集中在上夜班和乳腺癌之間的聯系上。而且他們都發現了上夜班的婦女比上白班的婦女得乳癌的機率要高得多——平均要高出48%。

科學家認為上夜班對人體的影響是由于它會干擾人體制造所謂的“黑暗荷爾蒙”——抗黑變激素。如果你整個晚上都使用人造光（即使是很微弱的人造光）你的腦袋就會停止生產抗黑變激素。而這種多任務荷爾蒙可以讓你的生物鐘持續運轉，保持你在白天的時候清醒，晚上的時候睡覺。它還在預防癌癥方面有不可小覷的功能。

抗黑變激素會降低女性血液里的雌激素——雌激素是公認的刺激某些癌癥生長的激素，比較顯著的例子是乳腺癌和子宮癌。抗黑變激素還會阻止癌細胞的增加，并且通過殺死破壞細胞的“自由基”（殺死自由基也是為什么抗氧化劑能受到如此巨大歡迎的原因）來提高人體的免疫系統能力，它還能防止癌細胞。大腦在晚間會產生最多的抗黑變激素，而當黑夜加深，上夜班的工人在人造燈的照射下時，抗黑變激素的水平就會不同尋常的低。

所以夜班工人應該怎樣擔心？“我想我們可以說這是一個比較大的公共健康問題，”安德魯沃特森教授說，他是斯特林大學的專業健康專家。我們并不僅僅只談乳腺癌（當然這個問題非常嚴重），他說，還有一些其他的健康問題與夜班工作有關系，包括前列腺癌或者子宮癌。夜班還可能對生育造成影響：丹麥通過對4萬個婦女的調查后發現，夜班工人比白班工人生的小孩的體重普遍要輕（而比較輕的小孩比較容易患上嚴重的健康問題，比如呼吸系統問題或者殘疾）。這些婦女的懷孕期還可以能持續較長的時間——這就增加了死胎的風險。

還有一些研究表明夜班與胃癌和腸癌也有關系——可能因為夜班工人吃得比較不健康而且鍛煉的也不夠造成的。沃特森說還有一些證據表明夜班與非霍其金淋巴瘤有關系，這個可能是因為身體的免疫系統缺少抗黑變激素的刺激造成的。“丹麥對于乳腺癌問題的回應是站在世界前列的，”他總結到。“英國政府絕對也需要這么做。”

英國政府現在還沒有計劃像丹麥政府一樣發放補償金。根據英國癌癥研究院所的看法，任何對于夜班的恐慌都還太早。“夜班工作導致婦女患乳腺癌的風險并沒有完全顯示出來，”博士Kat Arney說，她是英國癌癥研究院的高級科學信息主任。這就是為什么當你研究生活方式對癌癥風險的影響時有那么多復雜的因素需要考慮。“現在我們并不知道其他生活方式的影響，比如做HRT，肥胖，少要小孩或者喝酒，這些與夜班一起都會增加女性患乳腺癌的風險。”Arney說。

所有這些因素——比起抗黑變激素——更可能是造成癌癥的真實原因。例如，Arney解釋道，“我們知道乳腺癌在缺少活動的婦女身上是比較普遍的，因此如果夜班工人比起普通人來說，鍛煉的比較少，這也可以用來解釋為什么夜班工人患上乳腺癌的幾率會比較大。”

“我們將會從即將出版的健康和安全指南研究里知道英國婦女面臨的更多的風險，這個研究結論將在2011年出版。”她說。但對于腸癌、前列腺癌或者子宮癌患者來說，每一種類型的癌癥都只有一到兩個研究組在進行研究，所以只有在做了更多的調查以后我們才能明確的知道他們之間的聯系。“有更多的調查最好，”沃特森說，“不過很明顯夜班與一些主要的健康問題是有關聯的。”

如果研究的結果真的證實是由于“夜晚的燈光”的影響，那么如何處理就會成為對健康問題的主要挑戰。例如，在你與患癌癥有關聯的風險變得巨大之前，每周要減少多少輪班？沃特森說可以明確的說“你不必每天都冒著風險上夜班。這只是你輪班的一種形式，你可以周期性去做，不過要有規律。”

現今的研究表明如果兩者之間有關聯也需要一段較長的時間來證實——你需要有規律的上20年到30年的夜班以后才會大量增加你患上乳腺癌的可能。英國癌癥研究所說直到有更多的證據以前，夜班工人——跟其他人一樣——在發現任何異常后都應該去找醫生，配合他們的國民健康保險制度上所寫的條款進行治療。

沃特森認為需要組織更多的公共健康機構。“我們不是要明確夜班已經造成的普遍傷害是什么，而且我認為這種想法是錯的，”他說。“傷害已經存在，我們只是沒有看到，沒有去思考而已。”

夜班工人的故事：

“我是一個性情乖戾的，情緒不穩的隱士”

在ITN當記者意味著有規律的為期兩周的夜班輪換。十四天的夜班后我發現感到膽戰心驚的害怕，我很清楚的知道自己會變成一個睡眠不足的，性情乖戾情緒不穩的隱士。

在一個晚上的10點，我沒有像平時一樣喝一杯放松酒，而是洗了一個讓自己能清醒起來的澡，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點。

兩個晚上以后情況就會好點——不過你要記得在上床睡覺之前關掉通訊工具，關掉手機還有拔掉門鈴——還有你的鄰居不會自己闖進來。

晚上的辦公室是個比較怪異的地方。沒有玩笑，而且很難保持積極的心態。而且我沒有做任何鍛煉——因為我醒著的時候都在黑暗之中，不太可能冒險出去外面跑下步。

14天過完后，我都會眼圈發黑，胃部抽筋（我的消化系統可能變得混亂了），而且我的情緒也會變得很情緒化（“沒有人知道我有多累！”）需要發泄。