Night work: Death on the job?

5. March 2015

Working, while others are sleeping, is part of the medical routine – and unhealthy. A recent American study reinforces previous findings: whoever works at night has a higher risk of cardiovascular disease and lung cancer.

An international research team, as part of a prospective cohort study, investigated whether changing night shifts is related to increased mortality. This is an issue that affects many individuals. As many as 20% of people in the workforce in Western countries do shift work during their working lives.

The more night shifts – the higher the risk

The authors took in data from the Nurses’ Health Study (NHS) as a reference. The result: doing night shift work for more than five years increases general and cardiovascular-based death rates. This was found to be highest among nurses with more than 15 years of shift work. A link between night shift work and a higher general cancer mortality rate did not exist. However, doing night work for more than 15 years was associated with an increased lung cancer mortality rate. “These results support earlier evidence on the possible harmful effects of alternating night shift work on health and life expectancy”, says author Eva S. Scherhammer MD, Associate Professor of Medicine at Harvard Medical School.

The NHS cohort has existed since 1976. The participating nurses were interviewed about shift work in 1988. After excluding those who had already been found to be suffering from a cardiovascular disease or cancer, the researchers analysed data which has been acquired entirely from the NHS between 1988 and 2010 and was derived from 74,862 women. Compared with women who have never worked at night, all those who had done at least five years of night shift work had an increase in general mortality risk by 11%. Other factors such as smoking, age, alcohol consumption, physical activity, BMI and socio-economic status were taken into account.

Higher risk for ischemic heart disease

Significantly clearer is the damaging influence of night work on cardiovascular mortality risk. This was for women who had performed at least five years of shift work 19% higher than in the control group, for those having done more than 15 years of shift work the figure was 23% higher after taking the above-mentioned factors into consideration. The researchers analysed the risks for the most common cardiovascular diseases among the study population. It was found that the risk for ischemic heart disease was elevated when the women had worked more than five years doing night shift. With more than 15 years of night shift the risk was even higher. In a further analysis, the risk factors hypertension, hypercholesterolemia and type 2 diabetes were taken into account – which did not however significantly affect the association between night shift work and ischemic heart disease.

The link between night shift work – which breaks the circadian rhythm – and ischemic heart disease can be explained on the basis of various biological mechanisms. According to one review article shift work has various physiological effects. The nervous system is activated in a certain way, there are increased inflammatory responses and lipid and glucose metabolisms change. The result is an increased arteriosclerosis-risk.

Lung cancer risk increases – even for non smokers

With regard to cancers in general, the authors could find no increased risk of mortality. The researchers then put those cancers which were responsible for more than 200 deaths in the study population under the microscope. It was found that women who had accumulated more than 15 years of night duty had a moderately increased lung cancer mortality rate. Smoking was taken into account by the authors as a possible disturbance variable – the risk for non-smokers also being significantly higher.

The increased cancer risk can be ascribed to an anti-tumour effect involving the circadian system, and in particular involving melatonin. It enhances the immune response, acting as an antioxidant and as an anti-inflammatory. As early as 2007 night shift work was therefore already classified by the WHO as potentially carcinogenic.

Strengths and weaknesses: the study size and study period versus current data

The authors themselves point out several weaknesses of the study. Firstly, the information on shift work is derived from a data collection from 1988 and has not since been updated. In addition, the authors were not able to exclude the possibility that the data is incomplete: the authors fear that nurses who only worked night shifts did not provide their information – although it was data which the researchers had also sought to acquire. Nevertheless this changed the study results only slightly, since the authors used more stringent statistical criteria. In order to distinguish between lack of sleep and night work, they analysed data from women who had six to eight hours of sleep – the data is not shown in the study in detail.

In looking at strengths of the study, the authors point out: “It is one of the largest prospective cohort studies in the world, with a large proportion of night shift workers and a long follow-up period”. In addition, all study participants work in nursing – differing types of employment do not therefore act as a statistical interference factor.

For the authors it is still too early to draw practical conclusions in relation to everyday work of night shift workers. The roles of the length and intensity of night shift work and the interaction of shift schedules and individual conditions still need to be further investigated. The results could possibly be affected by the chronotype – the category according to which the internal biological clock of each person operates. After all, some people are more diurnally active, others nocturnally active.


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Dear Ms. Kaycoff,

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#2 |
Virginia Kaycoff
Virginia Kaycoff

Terrible syntax, multiple punctuation errors. Where do you find these writers?

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