Night work increases the risk of breast cancer in nurses

In a comprehensive study, the links between night work and breast cancer risk were investigated within a cohort of Norwegian nurses. Significantly, increased risks were seen in nurses who worked six or more consecutive nights for more than five years. This seemed to indicate that the risk of contracting breast cancer may be related to the number of consecutive night shifts. A relatively high proportion of people in Norway work night shifts, many of them in the healthcare sector.

Background

Breast cancer is the most common cancer among women in Western societies. In 2007, the International Agency for Research on Cancer (IARC) classified shift work that involves disruption of circadian rhythms as a probable human carcinogen (Stevens et al, 2011).

Although the association between the aspects of night work and breast cancer risk have not been sufficiently understood, it has been suggested that night work may influence cancer risk through sleep deprivation and depression of the hormone melatonin due to exposure to light at night (Stevens, 2009).

The proportion of workers who work shifts or nights is higher in Norway than in many other countries, and the scope of occupations affected has increased steadily in recent years. In 2008, 23% of workers said they worked shifts, with 16% working nights. The relatively high proportion of people working in the health and care sector probably contributes to this.

Over the past few years there has been increasing public concern in Norway about night work and its link to the risk of breast cancer. The objective of a recent study (Lie et al, 2011), based on a Norwegian cohort of nurses (Lie et al, 2007), was to examine the relationship between shift work – including night shifts – and breast cancer risk, looking at different exposure metrics of night work.

About the study

A case-control study was carried out within a cohort of 49,402 female nurses who graduated from a three-year nursing course between 1914 and 1985. It was established in 2004, on the basis of information from the Norwegian Board of Health Supervision (Helsetilsynet) registry of nurses.

First-occurrence breast cancer cases were identified by linkage to the Cancer Registry of Norway (Kreftregisteret) by the 11-digit personal identification number given to all Norwegians from 1960 onwards. Cases were included if diagnosed during the period from January 1990 to December 2007, aged 35–74 years at diagnosis, and alive by February 2009.

Altogether, 74% of the primary breast cancer cases identified using these criteria (699/943) and 65% of the controls (895/1,384) were interviewed and included in the study.

For the interviews, a structured questionnaire was used that included detailed questions on work and other known or potential breast cancer risk factors that might affect the association between night work and breast cancer risk.

A ‘night shift’ was defined as one that included the hours between midnight and 6am, although the shift may have started earlier or ended later. This is the normal night shift in Norway.

It was acknowledged that interview data could be subject to recall bias, with the risk of cases and controls reporting exposures differently on the basis of their status as either case or controls. In order to minimise the risk, the study was designed to investigate a broader spectrum of work-related factors that might also be related to breast cancer risk.

Key findings and discussion

In a previous study of Norwegian nurses (Lie et al, 2006), a significantly elevated risk was found among nurses who had worked for 30 years or more in hospitals and other institutions. This result was not replicated in the present study, when applying the same surrogate measure of exposure.

In the new study, risk estimates by ‘lifetime average number of night shifts per month’ were slightly, but non-significantly, elevated in exposed nurses compared with nurses who had never worked at night. When applying different, previously-used exposure metrics for night work, estimates showed a slight (10% to 30%) but non-significant increase of risk for breast cancer in the highest exposure categories.

However, when using consecutive night shifts as the exposure parameter, the risk was found to be 70–80% higher for nurses who had worked for more than five years, regularly working at least six night shifts in a row. The observed increase of risk for this group not only supports an association between night work and breast cancer, but also suggests that breast cancer risk may be related to the number of consecutive night shifts.

Commentary

As disruption of the circadian rhythm is assumed to be a primary pathway from shift work to disease, it is important to identify and implement shift systems that minimise circadian disruption and other factors that might be involved in carcinogenicity.

One strategy to diminish circadian disruption may be to minimise the number of consecutive nightshifts. The use of forward-rotating shift systems (1–2 consecutive night shifts) seems to be more favourable for sleep, performance, and the social life of workers than the use of more slowly rotating shift rotations (3–5 consecutive nightshifts). Another measure could be setting limits to the total ‘work at night’ demanded during one individual’s working life.

References

Lie, J.A. and Kjaerheim, K. (2007), ‘Cancer risk among 43,000 Norwegian nurses’, Scandinavian Journal of Work, Environment and Health, Vol. 33, No. 1, pp. 66–73.

Lie, J.A., Kjuus, H., Zienolddiny, S., Haugen, A., Stevens, R.G., Kjaerheim, K. (2011), ‘Night work and breast cancer risk among Norwegian nurses: Assesment by different exposure metrics’, American Journal of Epidemiology, Vol. 173, No. 11, pp. 1272–1279.

Lie, J.A., Roessink, J., Kjaerheim, K. (2006), ‘Breast cancer and night work among Norwegian nurses’, Cancer Causes Control, Vol. 17, No. 1, pp. 39–44.

Stevens, R.G. (2009), ‘Light-at-night, circadian disruption and breast cancer: assessment of existing evidence’, International Journal of Epidemiology, Vol. 38, No. 4, pp. 963–970.

Stevens, R.G., Hansen, J., Costa, G., Haus, E., Kauppinen, T., Aronson, K.J. et al (2011), ‘Considerations of circadian impact for defining ‘shift work’ in cancer studies: IARC Working Group Report’, Occupational and Environmental Medicine, Vol. 68, No. 2, pp. 154–162.

Bjørn Tore Langeland, National Institute of Occupational Health