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). Does being a morning or an evening person play a role in the risk of breast cancer? Researchers say that being a morning person can help avoid breast cancer. 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. Results showed that, given that the MR analysis confirmed the first set of results, the authors conclude that the findings provide strong evidence for a causal effect of chronotype on breast cancer risk.