ISLAMABAD-Could being a morning or an evening person be a risk factor for breast cancer? A large study suggests that women who are morning people could be at lower risk.

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

For their analysis, they used data on 180,216 women from the UK 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.

The researchers wanted to carry out the study because a lot of published research on breast cancer risk and sleep has tended to focus on “night shift work and exposure to light at night.”

Far fewer studies have focused on traits or personal attributes that individuals find much harder to change, if they can change them at all. A preference for morning or evening is an example of such a trait, which the authors describe as a ‘chronotype’.

They note that a number of “large genome-wide association studies” have generated robust genetic profiles for chronotype (i.e., morning or evening preference), sleep duration, and symptoms of insomnia.

In the new study, the researchers carried out two types of analysis. In the first type, they ran a multivariable regression analysis on the UK Biobank data to find links between breast cancer and what each participant reported as their morning or evening preference, sleep duration, and insomnia symptoms.

In the second type of analysis, they used participants’ genetic profiles of chronotype, sleep duration, and insomnia to look for links between these and breast cancer.