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. 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. 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. The team chose to use MR analysis because their data came from observational studies, which are studies that track people over time. Such studies can only find links between variables, they are not able to prove that one variable actually causes another. 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."