CORRESPONDENCE

Re: Night Shift Work, Light at Night, and Risk of Breast Cancer

Davina Porock, John Gentry

Affiliations of authors: D. Porock, University of Missouri Sinclair School of Nursing, University of Missouri-Columbia; J. Gentry, University of Wisconsin-Madison Comprehensive Cancer Care Center.

Correspondence to: Davina Porock, Ph.D., University of Missouri/Columbia, MU Sinclair School of Nursing, S428 Nursing Bldg., Columbia, MO 65211–2222 (e-mail: porockD{at}missouri.edu).

Our interest in the article "Night Shift Work, Light at Night, and Risk of Breast Cancer" (1) began with the media coverage of the research in a number of newspapers across the country. Shift work is already known to have a negative impact on health, work performance, and social life (25). If this research (1) is correct, then there is a further risk of malignancy, providing the shift work includes sufficient time on the night shift [no wonder Davis et al. (1) refer to it as the "graveyard shift"].

The study by Davis et al. (1) is based on 1) the link between exposure to estrogen and the risk of breast cancer and 2) the fact that exposure to light at night increases exposure to estrogen through mechanisms associated with the circadian rhythm. This is not an illogical train of thought. Four studies were cited [references within (1)], reporting an increased risk of breast cancer in women who work during the night. Of the four studies, three were conducted in Scandinavia, a geographic location known for its long nights in winter, and two focused on airline crews, who have greater exposure to cosmic radiation. Furthermore, in the "Discussion" section, Davis et al. (1) make reference to research that found women with bilateral blindness had a statistically significantly lower risk of breast cancer, thus supporting the notion that there is an association between light at night and breast cancer risk.

However, in Davis et al. (1), the data collection is based on recall of occupational history, sleeping habits, bedroom environment over a decade, and other known risk factors. To minimize the known limitations of recalled data, the manuscript would have benefited from a discussion of the reliability of the recalled data, the development of the questionnaire, any pilot work, and the training of the interviewers.

Table 3 in Davis et al. (1) reported the findings on occupational history—the one variable that is likely to have some reliability because, understandably, women who have worked the "graveyard shift" would remember the years of that employment. The major thrust of the authors' work is predicated on a small difference between the number of women among the case patients (n = 54 of 767 [7%]) and those among the control group (n = 37 of 743 [5%]) who had ever worked a night shift. Although the difference may be statistically significant in this analysis, the question is whether a 2% difference is clinically significant. A difference in the rates of breast cancer for night shift workers over and above that of other workers must be solidly established. The data displayed in Table 3 show that the study yielded 54 night workers in the case group and 37 in the control group. A simple {chi}2 test on these data does not reach statistical significance (P>.1).

There are many possible variables that have a relationship to the development of breast cancer that were not reported, for example, ethnicity or socioeconomic status (6) and physical exercise (7). In fact, economic status in particular may affect the need to work on the graveyard shift. A statistically significant difference between the case patients and control subjects was found for those having worked the graveyard shift for 5.7 years or more. This may indeed show a need to obtain more data on this interesting potential risk factor.

Questions regarding the risks to women for breast cancer are important, and the need for research that investigates all possible factors, particularly those that are potentially under a person's control, are essential. Nevertheless, the need for research with robust data is imperative if the scientific community is to ensure that women have access to the quality information that they deserve.

REFERENCES

1 Davis S, Mirick DK, Stevens RG. Night shift work, light at night, and risk of breast cancer. J Natl Cancer Inst 2001;93:1557–62.[Abstract/Free Full Text]

2 Fitzpatrick JM, While AE, Roberts JD. Shift work and its impact upon nurse performance: current knowledge and research issues. J Adv Nurs 1999;29:18–27.[Medline]

3 Karlsson B, Knutsson A, Lindahl B. Is there an association between shift work and having a metabolic syndrome? Results from a population based study of 27,485 people. Occup Environ Med 2001;58:747–52.[Abstract/Free Full Text]

4 Munakata M, Ichi S, Nunokawa T, Saito Y, Ito N, Fukudo S, et al. Influence of night shift work on psychologic state and cardiovascular and neuroendocrine responses in healthy nurses. Hypertens Res 2001;24:25–31.[Medline]

5 van Amelsvoort LG, Schouten EG, Maan AC, Swenne CA, Kok FJ. Changes in frequency of premature complexes and heart rate variability related to shift work. Occup Environ Med 2001;58:678–81.[Abstract/Free Full Text]

6 Baquet CR, Horm JW, Gibbs T, Greenwald P. Socioeconomic factors and cancer incidence among blacks and whites. J Natl Cancer Inst 1991;83:551–7.[Abstract]

7 Bernstein L, Henderson BE, Hanisch R, Sullivan-Halley J, Ross RK. Physical exercise and reduced risk of breast cancer in young women. J Natl Cancer Inst 1994;86:1403–8.[Abstract]


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