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.

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New research suggests that women who find it easier to wake up early in the morning may have a lower risk of developing breast cancer.

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.

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.

This second type of analysis is called Mendelian randomization (MR), and they ran this on the UK Biobank data and also two samples of Breast Cancer Association Consortium (BCAC) data.

The team compiled the genetic profiles from “341 single nucleotide polymorphisms (SNPs) associated with chronotype, 91 SNPs associated with sleep duration, and 57 SNPs associated with insomnia symptoms.”

The multivariable regression analysis of the UK Biobank data showed that women with a morning preference had a less than 1% lower risk of breast cancer, compared with women with an evening preference.

A factor that has a less than 1% effect on women’s breast cancer risk means that it affects fewer than 10 women in 1,000.

This first analysis found “little evidence for an association between sleep duration and insomnia symptoms.”

The MR analysis of the UK Biobank data supported these findings, as did the MR analysis on two samples of BCAC data, with one exception. This showed a small “adverse effect of increased sleep duration on breast cancer risk.”

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.

By using MR analysis and other methods, and ruling out known risk factors, the researchers sought to make their results more reliable and less disrupted by factors that they could not measure.

In other words, they took observational data as far as they could toward supporting that a cause-and-effect link exists, although that type of data does not comprise proof.

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.”

Commenting in a linked editorial, Eva Schernhammer, a professor in the department of epidemiology at the University of Vienna, writes that the study identifies “a need for future research exploring how the stresses on our biological clock can be reduced.”

In addition, she suggests that this type of study “could also help to align working hours with chronotype — to more closely match externally imposed timing with individual diurnal preference, especially in the working population.”

Dr. Luca Magnani, a senior research fellow in surgery and cancer at Imperial College London in the United Kingdom, also comments on the findings.

He describes the study as an “interesting” one in which “the researchers have identified a link between a genetic variant (genetic variants associated with three particular sleep traits) and breast cancer risk — which is significant, but a very small effect size.”

He also points out that the results in no way suggest that “modifying sleep habits could eventually lead to a decrease in the risk of breast cancer (there is no association with sleep habits or insomnia).”