Study finds link between women working night shifts and cancer. But how significant is the risk?

A recent widely reported medical study found that working the night shift increases the risk of cancer in women. But this is only half the story. Previous studies have shown a similar increase in the risk of cancer for men. The findings from all these bits of research might be particularly significant for India, which has a large labour force that works at night. Apart from truck drivers, security guards, nurses and factory workers, India has about 11 lakh BPO workers who work regular night shifts.

“Night shifts are a risk factor for the development of cancer,” said Dr Neelesh Reddy, consultant oncologist at Columbia Asia Referral Hospital in Bengaluru. “However, there is no evidence that one sex is more prone to cancer than the other. A lot more research needs to go into this, keeping in mind that our work culture today tends to be 24/7.”

The study establishing the link between women working night shifts and increased cancer risk was a meta-analysis of previous research and was published in January this year in the Journal Cancer Epidemiology, Biomarkers and Prevention. The researchers analysed 61 studies with data from close to four million people, of whom about 1,15,000 had cancer. The study found that long-term night shift work among women increased their risk of developing 11 common kinds of cancer. The risk was higher by 19% across these types of cancer - less for some and more for others. This analysis does not show comparative rates for men who work night shifts.

However, a study published in November 2016 in the Annals of Medicine reviewed the sleep habits of 27,000 male night shift workers in China and found that men who worked the night shift for more than twenty years or without resting during the day have an increased cancer risk of 27%.

Medical research does indicate that lack of sleep can increase our chances of getting cancer, but just how significant are these risks and how vulnerable are we? That risk is different for different populations, people of different age groups and varies based on exposure to environmental factors.

The study on women and cancer risk breaks down its findings showing that women working night shifts over extended periods of time are 41% more vulnerable to skin cancer, 32% more vulnerable to breast cancer and 18% more vulnerable to gastrointestinal cancers.

But even that increased risk depends on the baseline risk that women in a particular population and demographic have.

“To put it into perspective:, if your baseline risk of contracting breast cancer as an Indian women over 60 is 1% per year, then by working the night shift, it goes up to 1.4%,” said Dr Shona Nag, head of medical oncology at Jehangir Hospital in Pune who specialises in breast and cervical cancer. “A sixty year old is not likely to work night shifts. For younger women, the baseline risk of contracting cancer itself is even lower. Skin cancers are especially rare in India, so baseline risks are less than .001%.”

Based on this low baseline risk, women in India are unlikely to have a dramatic increase in risk of cancer because of working the night shift.

No good night’s rest

Why does working through the night increase the risk of cancer at all?

It starts off by triggering sleep deprivation in people working night shifts. “The circadian rhythm of body is affected when you sleep during the day,” said Reddy. The circadian rhythm is the body’s 24-hour internal clock that is attuned to the cycle of day and night. The circadian clock helps a person stay active during the daylight hours and results in waning energy levels at night. Night shift workers are constantly fighting against this body clock and find it difficult to get quality sleep during the day. As a result, they are often sleep deprived.

Such sleeplessness exacerbates conditions in the body that could lead to the development of cancer. According to Nag, chronic sleeplessness can lead to obesity by impacting secretion of hormones that increase appetite and that indicate when the body is satiated. These hormonal changes can lead to increased food intake without the compensating energy expenditure. An obese person experiences changes in body fat distribution that makes him or her more prone to cancer.

“Sleep deprivation is also known to lead to the inflammation of the body,” said Nag. “When the body continues to be in the inflammatory state, it produces cytokines, which research has proven to induce cancer over a long period. A lack of sleep can potentially cause the development of prostate, lung and breast cancer.”

Moreover, studies have established a link between constant disruption of circadian rhythms and disruptions in the production of the hormone melatonin, Melatonin both induces sleep and has a protective effect against cancer. These findings led the World Health Organisation’s International Agency for Research on Cancer in 2007 to classify shift work with circadian disruption as a probable human carcinogen that affects both sexes.

More recently, in a study published in the journal Nature in January 2018, researchers from the Salk Institute found through animal studies that activating two hormone receptors in the body known to be essential to establish circadian rhythm killed cancer cells. Hormone receptors are special proteins found on the surfaces of cells. These proteins constantly receive messages from our bloodstream and convey these messages to the cells. In effect, hormone receptors act as an on-off switch for any activity in the cells. Researchers found that good sleep was a cue for these two particular hormone receptors to prevent the formation of cancer cells.

Such findings indicate why a disruption in circadian rhythm can increase risk to cancer but also why people with cancer often find it very difficult to sleep. “It’s a vicious cycle,” said Nag.

Taking precautions

Working the graveyard shift remains a risk factor for cancer but it is impossible to do away with night shift work in our societies. So how does one cope?

“Anybody working odd hours must aim to get as much sleep as they can on holidays and weekends to mitigate these risks,” said Nag. “It is important is to ensure that one gets six to eight hours of sleep, be it in the day or in the night. As long as it is continuous sleep and has the usual sleep cycles, it will be able to mitigate the risks caused by not sleeping at night.”

While this is an effective short-term compromise for night shift workers, many experts say that it is impossible, especially in the long-term, to sleep during the day as deeply as we do at night.

Lack of sleep is only one of many risk factors for cancer, Reddy pointed out. “If people avoid other risk factors like tobacco, alcohol, fast food, obesity and follow healthy food and lifestyle practices the risk can be reduced,” he said.

Even avoiding all known environmental factors is no guarantee against cancer. At the same time, getting enough sleep is always a good idea to keep obesity, cardiac disease, stroke, and psychiatric disorders at bay.