Good news, morning larks – research shows that women who feel at their most awake in the hours before lunch are 40 percent less likely to develop breast cancer than late risers.

Scientists at Bristol University, UK, have found that something as simple as your internal body clock (or circadian rhythm) can play an important role in how likely (or not) you are to be diagnosed with cancer. The team presented their findings at the NCRI Cancer Conference in Glasgow, Scotland, while their paper, published on bioRxiv, awaits peer review. 

Each of us has a circadian rhythm that is unique to us. It follows a roughly 24-hour cycle and can change within a lifetime – elementary school children tend to be morning larks and teenagers are usually night owls, for instance. By the time we reach adulthood, it settles and for most of us, it falls somewhere in between the two extremes. 

Recently, studies have shown our circadian rhythm may influence everything from our procrastination habits to our drinking habits to our ability to play baseball. While morning larks may have the upper hand when it comes to health and (possibly) happiness, it looks like night owls can take home the prize for being smarter, more creative, and more prolific lovers.

This new study reveals another benefit to being a morning person – lower breast cancer risk.

The team used a method called "'Mendelian randomization", which uses genetic variants linked to potential risk factors (in this case, circadian rhythm) to determine whether or not there is a causal relationship between the risk factor and a particular disease (in this case, breast cancer). Using genetic variants reduces the effect of confounding or reverse causation, making the technique more reliable than other observational methods. 

In total, 341 variants linked to circadian rhythm and no other known risk factors for cancer (for example, obesity) were analyzed. DNA samples came from more than 220,000 women in the Breast Cancer Association Consortium (BCAC) study and more than 180,000 women involved in the UK Biobank project. 

The samples from BCAC showed that those with lark variants had a 40 percent lower risk of developing breast cancer than those with night owl variants. What's more, every additional hour slept after the recommended eight-hour sleep was associated with a 20 percent increase in risk.

The Biobank samples did not find a link between sleep duration and breast cancer risk but they did confirm that larks have a lower risk of developing the disease. During an eight-year period, approximately two in 100 night owls were diagnosed with breast cancer. Among morning larks, that figure was roughly one in 100. 

If the study passes peer review, it will show convincing evidence linking body clock to breast cancer risk but it does not explain why this link exists in the first place. Is it tied to genetics or is it a lifestyle factor? 

"We would like to do further work to investigate the mechanisms underpinning these results, as the estimates obtained are based on questions related to morning or evening preference rather than actually whether people get up earlier or later in the day," Rebecca Richmond, a research fellow in the Cancer Research UK Integrative Cancer Epidemiology Programme and the MRC Integrative Epidemiology Unit at the University of Bristol, explained in a statement.