Did you know that working night shifts is classed as a carcinogen, according to the World Health Organisation (WHO)? Do you know which country became the first to award worker compensation to women who developed breast cancer after years of working unsociable hours in government-sponsored jobs, such as nurses or cabin crews? 

Which enlightened nation do you think this was? Was it the USA, the beacon of hope and freedom, or maybe Russia returning to give succour to the proletariat? No, it was neither of these world powers. It was the diminutive nation of Denmark which has led the world in its moral obligation to provide compensation to its citizens who suffered the ravages of cancer as a consequence of late-night work schedules. If you live in the UK, to date, the British government has resisted similar legal claims and refuses to pay compensation. 

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Denmark compensates night shift workers who develop breast cancer.

Women in Denmark who developed breast cancer after many years of working night shifts have received compensation.1 Out of the 78 cases reported to the National Board of Industrial Injuries in Denmark, 38 received compensation through their employers’ insurance schemes. These women had worked night shifts for at least 20 years and had low-risk factors such as low alcohol consumption and no family history of breast cancer. 

The evidence prompted the World Health Organization to include circadian rhythm disruption as a probable carcinogen. A ruling from the International Agency for Research on Cancer in December 2007 said that “shift work that involves circadian disruption is probably carcinogenic to humans.” The agency, a part of the World Health Organization, classified shift work as a group 2A cancer risk.2

This ruling led the Danish government to compensate shift workers with breast cancer. It remains to be seen if other countries follow Denmark’s enlightened example.

Circadian rhythm disruption and cancer

We’re only now beginning to grasp the profound and enduring consequences of regular disruption of the circadian rhythm. Exploring the health of individuals engaged in prolonged shift work offers valuable insights into this phenomenon. For more than two decades, we’ve recognised some potential risks. A 1996 study hinted at elevated rates of breast cancer among Norwegian radio and telegraph operators, a finding that has since been confirmed on many occasions.3 Large-scale studies have shown that female shift workers have a 60% increased risk of breast cancer4  and a 35% greater risk of colorectal cancer.5

Does melatonin have any impact on getting cancer?

Shift workers may experience increased rates of certain cancers because of exposure to light at night (LAN). This exposure suppresses the pineal gland’s production of melatonin, which is believed to possess anti-cancer properties in addition to its sleep-inducing actions. It was once thought that this hormone, called the Dracula hormone since it only emerges at night, was solely responsible for ushering us into the land of nod. However, researchers have discovered it has several properties beyond its nocturnal hormonal functions. What’s exciting to researchers is its cancer-fighting abilities,6  and its antioxidant properties, allowing it to absorb toxic by-products of oxygen metabolism7. These by-products are thought to cause DNA damage and increase cancer risk. Therefore, it’s been speculated that by bathing ourselves in artificial light during night shift work, we prevent the release of melatonin, depriving our body of its protective shield and possibly contributing to the higher incidence of cancer in late-night shift workers. 

Breast cancer rates in blind women

So, by regular exposure to light at night (LAN), perhaps we’re lessening our resistance to cancer. This hypothesis is supported by the fact that people who are totally blind are less likely to develop breast cancer than normally sighted individuals. The risk of breast cancer in visually impaired women has been investigated across four nations: Finland, Norway, Sweden, and the United States. In each study, a decreased risk was observed among blind women, with reductions ranging from 20% to 50%.  If we presume that the reduced risk of breast cancer among blind women is indeed linked to light exposure among sighted women, then in that case, the projected number of breast cancer cases attributable to light at night (LAN) is quite substantial.8

Sleep and the immune system

Do you remember the last time you had the flu? It was probably not a very pleasant experience. What was the one thing you wanted to do? Was it jumping out of bed and running a marathon? Probably not; any form of physical activity was probably the last thing on your mind. You probably wanted to crawl under the covers, hide away from the world and get some sleep. Was your body trying to tell you something? Why are you programmed to seek sleep when you feel ill? What is happening during sleep that doesn’t happen when you’re awake? I’ll give you a big clue: it concerns your immune system.

The immune system is activated during sleep

When you sleep, your immune system is active, and everything is shut down to prioritise its nocturnal actions, from fighting infections to dispatching natural killer cells to hunt down any cancers. The immune system actively stimulates sleep when we fall ill because it knows something you don’t. You heal when you sleep. The immune system knows that sleep is the perfect environment to conduct its housekeeping role, free from the distractions of the daytime hustle and bustle.

Sleep deprivation and the common cold

Dr Aric Prather carried out an interesting experiment at the University of California, San Francisco, to determine the effect of sleep on the immune system. This involved him measuring the sleep of 150 volunteers, after which he quarantined them. Then, as any good host would, he shared something with them. Unlike usual dinner hosts who provide their guests with the choicest morsels of food, Dr Prather squirted the rhinovirus (common cold virus) up the volunteer’s noses instead. He wanted to see if there was a correlation between the number of hours the participants slept one week before getting squirted with the virus and becoming infected with a full-blown cold. He found that for those sleeping five hours on average, the infection rate almost reached 50%, while for those who slept seven hours or more, the infection rate dropped to 18%.9

Sleep deprivation reduces natural killer cells

So, sleep deprivation can severely impact our immune system, and if we return to the cancer-fighting abilities of the immune system’s special forces, the natural killer cells,  researchers led by Dr Michael Irwin at the University of California found that a single night of sleep of only four hours reduced the number of circulating natural killer cells by 70%.10

The inflammatory state and cancer

In addition to what’s been mentioned regarding the link between sleep disruption and cancer, another confounding factor is becoming more widely known. That’s the role of the inflammatory process in the growth of cancers. Sleep disruptions stimulate the sympathetic nervous system; contrary to what the name suggests, this part of the nervous system is not sympathetic; instead, it is the raging monster that stimulates the “fight or flight” reaction. Having the sympathetic nervous system turned up to the max by sleep disruption stimulates the inflammatory process, leading to a state of chronic inflammation. 

Cancers exploit this environment of chronic inflammation. Some cancers use the inflammatory state to facilitate the growth of new blood vessels, called angiogenesis, to steal vital nutrients from the body. Other cancers use the inflammatory factors to further damage and mutate their cancer cells, while others use them to dislodge themselves from their moorings, encouraging metastasis, i.e. moving to another part of your body.

Sleep-deprived mice suffered a 200% increase in the speed and size of cancer growth

To further study the link between cancer and lack of sleep, Dr David Gozal from the University of….no it is not California this time but the University of Chicago, injected mice with malignant cells and observed the tumour progression in relation to sleep. He found that the sleep-deprived mice suffered a 200% increase in the speed and size of cancer growth.11

Sleep is the best defence against cancer

These are sobering facts given the widespread prevalence of cancer, and yet we still pay little heed to one of the best defences against cancer: sleep. The ever-hungry capitalist world requires us to live in a 24-hour society, with many people being forced to work graveyard shifts, which are literally becoming graveyard shifts.

The evidence is mounting about the dangers of shift work, and we can only hope that more governments adopt Denmark’s enlightened approach, admit the dangers to health, and adequately compensate workers who have sacrificed their health.


  1. Wise J. Danish night shift workers with breast cancer awarded compensation. BMJ. 2009 Mar 18;338:b1152. doi: 10.1136/bmj.b1152. PMID: 19297443.
  2. Straif K, Baan R, Grosse Y, et al. Carcinogenicity of shift-work, painting, and fire-fighting. Lancet Oncol. 2007;8:1065–1066.]
  3. Tynes T, Hannevik M, Andersen A, Vistnes AI, Haldorsen T. Incidence of breast cancer in Norwegian female radio and telegraph operators. Cancer Causes Control. 1996;7(2):197–204.
  4. Davis, S., Mirick, D.K. and Stevens, R.G., 2001. Night shift work, light at night, and risk of breast cancer. Journal of the national cancer institute, 93(20), pp.1557-1562.
  5. Schernhammer, E.S., Laden, F., Speizer, F.E., Willett, W.C., Hunter, D.J., Kawachi, I., Fuchs, C.S. and Colditz, G.A., 2003. Night-shift work and risk of colorectal cancer in the nurses’ health study. Journal of the National Cancer Institute, 95(11), pp.825-828.
  6. Li, Y., Li, S., Zhou, Y., Meng, X., Zhang, J.J., Xu, D.P. and Li, H.B., 2017. Melatonin for the prevention and treatment of cancer. Oncotarget8(24), p.39896.
  7. Reiter, R.J., Mayo, J.C., Tan, D.X., Sainz, R.M., Alatorre?Jimenez, M. and Qin, L., 2016. Melatonin as an antioxidant: under promises but over delivers. Journal of pineal research, 61(3), pp.253-278.
  8. Stevens, R.G., 2002. Lighting during the day and night: possible impact on risk of breast cancer. Neuroendocrinology Letters, 23, pp.57-60.
  9. Aric A. Prather, Denise Janicki-Deverts, Martica H. Hall, Sheldon Cohen, Behaviorally Assessed Sleep and Susceptibility to the Common Cold, Sleep, Volume 38, Issue 9, September 2015, Pages 1353–1359,
  10. Irwin, M., McClintick, J., Costlow, C., Fortner, M., White, J. and Gillin, J.C., 1996. Partial night sleep deprivation reduces natural killer and celhdar immune responses in humans. The FASEB journal, 10(5), pp.643-653.
  11. Hakim F, Wang Y, Zhang SX, Zheng J, Yolcu ES, Carreras A, Khalyfa A, Shirwan H, Almendros I, Gozal D. Fragmented sleep accelerates tumor growth and progression through recruitment of tumor-associated macrophages and TLR4 signaling. Cancer Res. 2014 Mar 1;74(5):1329-37.

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