Shift work is hard on the body. It’s a schedule that reprograms the biological clock every few days. Those adjustments can disturb sleep patterns, impair mental acuity, and foster
irritability. In fact, it might be even worse than that. Two new studies find evidence that women who work the graveyard shift also increase their chance of developing breast cancer.
Both reports, published in the Oct. 17 Journal of the National Cancer Institute, raise the prospect that the increased risk results from chronic suppression of melatonin. Concentrations of this brain hormone normally peak during darkness, usually around 1 a.m. Previous studies have indicated that in animals, nighttime lighting–which suppresses melatonin release–boosts the growth of cancers (SN: 10/17/98, p. 248: http://www.sciencenews.org/sn_arc98/10_17_98/19981017fob.asp.).
In the first of the new studies, the working hours of 800 Seattle-area women with newly diagnosed breast cancer were compared with those of an equal number of healthy women their age. Scott Davis of the Fred Hutchinson Cancer Research Center in Seattle and his colleagues also interviewed each woman about her sleeping habits throughout the previous decade.
Among the 1,600 women, only 11.4 percent reported that commonly they weren’t asleep during the period around 1 a.m. Davis found that women with breast cancer were more likely to have been among those who sometimes slept at atypical times. Half of the woman who slept during odd hours periodically worked at night. Women who averaged at least 5.7 hours of night work each week faced double the risk of developing this cancer compared with women who didn’t work nights. The study reported “clear evidence of a trend of increasing [cancer] risk with increasing years of graveyard shift work.”
In the second study, Eva S. Schernhammer and her colleagues at Brigham and Women’s Hospital in Boston analyzed data from 78,500 nurses taking part in a long-running health study. Some 2,400 developed breast cancer during the decade ending in 1998. The researchers compared the work history of these women with that of women who remained cancerfree.
Overall, 60 percent of the nurses reported at least occasional shift work. Risk of breast cancer increased with the years of shift work. Those who worked some nights for at least 30 years faced a 36 percent higher risk of breast cancer than those who never worked at night.
The two studies point out “an urgent need” to further explore links between light at night and cancer, argues Johnni Hansen of the Danish Cancer Society in Copenhagen. Indeed, he notes in an accompanying editorial, these and “apparently all [other] epidemiological studies published so far on different indirect measures of light at night and breast-cancer risk seem to relatively consistently point to an increased risk.” Of occupational factors that have been at least tentatively linked to cancer, working at night is the most common, he notes.