The researchers describe their methods and findings in a recent BMJ paper.

For their analysis, they used data on 180,216 women from the UK Biobank and 228,951 women from the Breast Cancer Association Consortium.

They report that they found "consistent evidence" of morning preference having a "protective effect" on breast cancer risk.

They also found "suggestive evidence" that sleeping more than 7–8 hours per night could have an "adverse effect" on breast cancer risk.

The researchers emphasize that the effects they found are small, compared with that of other risk factors for breast cancer, such as BMI, alcohol consumption, and smoking.

Breast cancer starts in breast tissue. It arises when abnormal cells grow out of control, invade nearby tissue, and spread to other parts of the body. Although it mostly strikes women, men can also get breast cancer.

In 2016, there were around 3.5 million women living with breast cancer in the United States, according to the National Cancer Institute (NCI), which is one of the National Institutes of Health (NIH).

The NCI also estimate that around 13% of women in the U.S. will receive a diagnosis of breast cancer at some point in their lives.

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