In a prospective study of 23 995 Japanese women, short sleep duration was associated with higher risk of breast cancer (143 cases), compared with women who slept 7 h per day, the multivariate hazard ratio of those who slept ⩽ 6 h per day was 1.62 (95% confidence interval: 1.05–2.50; P for trend=0.03).
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Kakizaki et al. (2008) studied this question.
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