Research published in The BMJ shows there is consistent evidence for a protective effect of morning preference and suggestive evidence for an adverse effect of increased sleep duration on breast cancer risk. Experts weigh in on Science Media Centre.

Prof Chris Bunce, Professor of Translational Cancer Biology, University of Birmingham, said:

“This study is impressive in its scale. However, the associations observed are very small correlative. It is dangerous to suggest, even unintentionally, to women that changing their sleep patterns will significantly alter their risk of breast cancer.

“The authors acknowledge that a limitation of their study is that the groups in the study bias the analysis to women of European ancestry. However, they do not consider that women included in the UK Biobank and BCAC are themselves a biased sample of women from the UK; first by virtue have having been eligible and second having volunteered to these studies.

“The study reports that of the women with an exclusive diagnosis of breast cancer in the UK biobank, approx. 60% were prevalent cases, which I take to mean had breast cancer at the time they entered the study and approx. 40% developed breast cancer after entering the study, half of whom developed cancer within just three years of entering the study. If this is correct, then most of the women either had cancer at the time of the study or probably had undetected early disease. In this case self-reported sleep patterns and other non-genetic measures taken at time of study entry may not be the same as prior to developing cancer which is when the primary risk occurs.”