The Big Question: Does working at night cause cancer, and should shift patterns be changed?

Why are we asking this now?

It emerged yesterday that the Danish government has begun paying compensation to women who developed breast cancer after long spells working at night. They are believed to be the first compensation payments by a Government for cancer attributed to shift work. Almost 40 women have so far received the payments.

Does night work cause cancer?

The International Agency for Research on Cancer (IARC) based in Lyon, France, concluded in 2007 that women who worked nights were at increased risk of breast cancer. The research was based on a review of epidemiological studies among nurses and flight attendants and found that "circadian disruption" of sleep patterns was "probably carcinogenic". The results were backed by animal studies showing that constant light, dim light at night or simulated jet lag substantially boosted tumour development. It was this conclusion by the IARC that led the Danish government to make the compensation payments.

How big is the risk?

The authors of the IARC review said it was "less than a twofold increase in risk" which they described as "modest". An earlier US study conducted by Harvard researchers found a 50 per cent increase in breast cancer among night-shift workers. One of the reports reviewed by IARC found a 36 per cent increased risk of breast cancer in women who had spent 30 years working nights compared with those who had never done so.

Are there other effects?

Yes. The most obvious one is fatigue. Most shift workers have a permanent sleep debt. Other problems include digestive disorders and an increased risk of accidents. Several studies have shown that shift workers have higher rates of heart disease and gastrointestinal disease. Some doctors have claimed that night work increases stomach ulcers and diabetes. According to one theory humans are programmed to eat during the day. Eating at night means we do not process the food in the same way. The implication is that we may be more exposed to carcinogens in our diet.

What is the cause?

A body clock that is out of phase with the environment is thought to be harmful. This is a common experience for travellers who cross time zones who find themselves pole-axed by jet-lag. For shift workers, used to night work, the harms are different. Altering sleep patterns affects the production of the hormone melatonin and disregulates genes that control tumour development. Melatonin, which is produced at night, helps regulate sleep and is often taken, as a supplement, by long-haul travellers. Women with high levels of melatonin in the morning have a lower risk of breast cancer. It is thought that exposure to artificial light among shift workers may boost production of oestrogen, the female hormone, which in turn suppresses production of melatonin. A rise in oestrogen may induce hormone sensitive tumours in the breast.

What is the cause of the circadian rhythm?

Sleep is controlled by separate drives. The body's biological clock generates and maintains circadian rhythms in temperature, blood pressure, sleep and wakefulness, mental performance and the synthesis of certain hormones. The circadian rhythm is regulated by light. The homeostatic drive, which works alongside it, is simpler and is determined by how much sleep we have had (or missed) – the longer we have been awake, the more we need to sleep. Trying to sleep at the "wrong" phase of the circadian cycle will usually lead to a disturbed "night" with frequent awakenings, resulting in the exhaustion seen in jet lag and among shift workers.

How widespread is night working?

Nine-to-five is giving way to the 24-hour society. Call centres are open permanently and large sections of the retail and entertainment industries operate around the clock. So do transport and communication industries. Nearly 20 per cent of the working population in Europe and North America is engaged in shift work. In Britain, more than one million women are estimated to be working during the hours of darkness. Many spend only brief periods working at night, but for an increasing number working unsocial hours is routine.

Will the Danish move set a precedent?

It will increase pressure on the UK. Professor Andrew Watterson, an occupational health specialist at the University of Stirling, said the UK was behind Scandinavia in recognising the dangers. "We don't tend to identify the damage being done where shift working is prevalent and I think that's an error," he told BBC Radio Scotland. "The damage is there but we don't see it and we don't count it." However, a spokesperson for the Health and Safety Executive said the link between breast cancer and shift work was "not that compelling" and it was too early to say what steps the British government might take. The HSE commissioned a report from Oxford University in 2007 into the possible health risks which is due for completion 2011.

Could there be further litigation?

Doctors have warned for a decade that increased shift work and the problems it brings could result in litigation unless employers help workers adapt their circadian rhythms. In one case in the US, the family of a woman killed in a car accident received a $24m settlement from the driver's employers after lawyers argued the company had violated regulations governing hours of work. The Danish experience shows that health problems could increase legal pressure on employers to reduce risks. In Britain, working times regulations introduced in 1998 limited hours of work, but did not take account of the effect of shift patterns on mental alertness. Carefully timed exposure to light could help, research suggests.

Can drugs help night workers adjust?

Yes. Hypnotics, drugs to help people sleep, are the second biggest-selling category after painkillers, with annual revenue of over $5bn. Researchers have been working on cures for jet lag, and drugs to help shift workers adjust to time changes. The drug tasimelteon is the latest to show promise in jet lag. A study by Harvard Medical School researchers published in The Lancet in December found it helped volunteers get to sleep and stay asleep longer after their sleeping pattern was disrupted to mimic long distance travel.

What about other treatments?

The biggest success of recent years has been Provigil, a drug to boost wakefulness, originally approved for the treatment of narcolepsy – a condition characterised by sudden attacks of daytime sleepiness, that cause collapse. It has since been cleared in the US for the treatment of night shift workers with excessive sleepiness. The military has also taken a close interest in the drug and its successors, some of which have shown even better results. But half of sales of Provigil are for "off-label" uses, indicating a high demand from healthy people for an alertness booster, otherwise known as a smart drug. As pressures in the modern world increase, the demand for substances that can tailor sleep to our needs is likely to grow.

Should people avoid working at night?

Yes...

* There is a higher risk of breast cancer and of digestive disorders, heart disease and other conditions

* Shift working causes problems with the body clock leading to fatigue, disrupted sleep and an increased risk of accidents

* It is anti-social and a cause of domestic difficulties

No...

* About one in five workers in Britain and Europe are engaged in shift work of some kind

* The link between shift work and cancer remains unproven, according to UK officials

* Drugs are available to help shift workers adjust to the time changes, and to promote sleep.