By Sara Freeman, medwireNews Reporter
There is no association between self-reported sleep duration, sleep quality, insomnia, or sleep disturbance and the incidence of breast cancer, according to findings from the Women’s Health Initiative.
While there was a trend for women who slept for longer durations than others to be more likely to be newly diagnosed with estrogen receptor (ER)-positive breast cancer, the association was weak and nonsignificant, the researchers report.
“These null findings are in agreement with the results of two previous cohort studies, the Nurses’ Health Study in the United States and the Finnish Twin Cohort, and one case-control study conducted in Australia,” Emily Vogtmann (University of Alabama at Birmingham, USA) and team report in Sleep.
The present study included data from 110,011 postmenopausal women aged from 50 to 79 years who were living in the USA, had no prior history of cancer, and provided information on sleep variables at baseline and at follow-up 1 to 3 years later. There were a total of 5149 incident cases of breast cancer identified after 1,190,565 cumulative person–years of follow-up.
In minimally adjusted models there was an inverse association between sleeping for longer periods (7 vs 5 or 6 hours) during the night and the incidence of breast cancer, but these associations were no longer significant after adjusting for potential confounding factors.
When ER status was considered, the hazard ratio (HR) for incident breast cancer in women who slept for 5 hours or less a night was 0.92, whereas it was 1.06 for women who slept for 9 hours or more a night; neither association was statistically significant.
“The weak positive association between sleep duration and risk of ER positive breast cancer in our study was unexpected,” the researchers observe. This is because lower levels of melatonin, which usually peak during the night, have been shown to correlate with higher estrogen levels. In turn increased estrogen exposure has been linked to a greater likelihood for ER-positive breast cancer, they explain.
A surprising finding, after stratification by body mass index, was that women of normal weight who suffered from insomnia but who never used sleep aids were less likely to develop breast cancer than those who did not have insomnia, at an HR of 0.92.
“It is possible that insomnia and breast cancer share a common cause that increases the risk of insomnia and decreases the risk of breast cancer,” the researchers propose. Whatever the reason, this finding “warrants further investigation,” they conclude.
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