Women Who Work Predominantly at Night: Bca Risk

Increased Breast Cancer Risk among Women Who Work Predominantly at Night

Johnni Hansen

Irregular working hours, including working at night, have serious psychological and physiological effects. In a nationwide population-based case-control study, we investigated the breast cancer risk among 30- to 54-year-old Danish women who worked predominantly at night.

Individual employment histo-ries were reconstructed back to 1964 for each of 7035 women with breast cancer and their individually matched controls from the records of a nationwide pension scheme with com-pulsory membership. Odds ratios, including 5 years of induc- tion time and adjusted for socio-economic status, age at the birth of first and last child and number of children, were estimated by conditional logistic regression analysis.

The odds ratio for breast cancer among women who worked at night at least half of a year was 1.5 (95% confidence interval, 1.2 to 1.7), and there was a tendency to increasing odds ratio by increasing duration of nighttime employment. (Epidemiology 2001;12:74 –77)

Working irregular hours, including night and shift work, has been acknowledged as an occupational health and safety problem since a century ago, as it has serious social, psychological, and physiological effects.1–3 Acute problems such as drowsiness and accidents have been studied most intensively 1 ; chronic health problems like gastrointestinal and coronary heart diseases are also rel-atively well described, although the mechanisms are not fully understood.2–6

It has been suggested that women with irregular working hours are at increased risk for breast cancer,7 since work that requires the use of arti-ficial light (in the evening, night, or early morning) leads to suppression of pineal secretion of melatonin, which may induce continuous production of estrogen involved in breast carcinogenesis.8

Irregular work schedules have become increasingly more common in most industrialized societies: in 1980, approximately 26% of men and 18% of women in the United States labor force worked variable shift sched-ules. 2 About 20% of the female Danish workforce works at night.9

We conducted a case-control study to inves-tigate whether women in Denmark who work predomi-nantly at night have an increased risk for breast cancer.

Subjects And Methods


Altogether 7565 women with confirmed primary breast cancer, born in the period 1935–59 and 30–54 years of age at the time of diagnosis, were identified in the files of the Danish Cancer Registry, which contains information on diagnosis (ICD-7), names, sex, and unique personal identification number.10

Information on past employment was reconstructed by record linkage with the files of the nationwide pension fund, adherence to which has been compulsory for all wage-earners in Denmark aged 18–66 years since its establishment in 1964. The fund retains computerized information about each job held, including the name and 10-digit personal identification number of the employee, the dates of start and end of employment and the company name and unique company number of the employer.11

This information is kept even after the employees have retired or died. No employment records existed for 530 of the breast cancer patients (7%), who were never part of the workforce (mainly housewives and assisting farmers’ wives), leaving 7035 female breast cancer patients who had an employment history.


One control subject per case was drawn at random from the files of the Central Population Registry, iden-tified by name, sex, and personal identification number, and matched to the cases on year of birth and sex. Controls had to be alive without cancer and to have been an employee (member of the national pension fund) before the date of diagnosis of the corresponding case.

The employment histories of the control subjects were retrieved from the files of the pension fund in the From Danish Cancer Society, Institute of Cancer Epidemiology, Strandboule-varden 49, DK-2100 Copenhagen Ř, Denmark.

Address correspondence to: Johnni Hansen, Danish Cancer Society, Institute of Cancer Epidemiology, Strandboulevarden 49, DK-2100 Copenhagen Ř, Denmark.

2001 article

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Czech Republic Outlaws Excess Outdoor Light

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Colorectal Ca & Night Shift Work: Women

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