Why might ‘morning people’ have lower breast cancer risk? 

A new study has identified five proteins in the blood that may help explain why people who are naturally ‘morning people’ tend to have a lower risk of breast cancer than ‘evening people’.

This work, led by researchers at the University of Hong Kong, was funded by Wereld Kanker Onderzoek Fonds, (WKOF) – the Netherlands-based charity within the World Cancer Research Fund network of charities.

The findings, published in the journal Communications Medicine, do not mean people can lower their risk simply by getting up earlier. Chronotype – our natural leaning towards mornings or evenings – is largely set by our genes and is hard to change. Instead, the real value of this research lies in the proteins themselves, which could point scientists towards new ways to prevent or treat breast cancer.

What is chronotype, and why does it matter for cancer risk?

Chronotype describes whether someone is naturally more of a ‘morning person’ or an ‘evening person’. Earlier research has linked having an evening chronotype, and other disruptions to the body’s internal clock – such as shift work, jet lag or irregular sleep – to a higher risk of breast cancer. But exactly why this happens has not been clear, which has made it hard to know what, if anything, could be done about it.

How did the researchers investigate this?

To dig into the biology behind the link, the team used a method called Mendelian randomisation. This uses genetic information to help work out whether one thing – in this case, chronotype – actually influences another, such as breast cancer risk, rather than the two simply appearing together by chance.

The researchers drew on genetic and health data from more than 650,000 people who took part in two large studies, UK Biobank and 23andMe, all of whom had self-reported whether they saw themselves as more of a morning or evening person.

They then took a two-step approach. First, they looked for links between chronotype and levels of nearly 5,000 different proteins in the blood, and found 895 proteins connected to a morning chronotype. Second, they checked whether any of those proteins were also linked to breast cancer risk and at this point, seven stood out as promising candidates.

To avoid false leads, the team ran an additional check called colocalisation. This statistical test shows whether the same genetic signal is genuinely influencing both the protein level and breast cancer risk, rather than two separate signals sitting close together by coincidence.

Dr April Shan Luo, Research Assistant Professor at Li Ka Shing Faculty of Medicine, the University of Hong Kong, who led the study, said:

“This gap in our understanding is significant in light of the growing prevalence of sleep deprivation in modern society and the challenges of being able to change or alter chronotype, particularly due to external factors such as shift work and frequent travel.

“There is a real need to better understand the link we are seeing between chronotype and breast cancer risk.”

What did they find?

After all the checks, five proteins stood out as having a genuine role in linking morning chronotype to a lower risk of breast cancer:

• ADAM15
• BTN2A1
• CASP8
• PDCD6
• RSPO3

Some proteins were more strongly linked to particular types of breast cancer than others. PDCD6, for example, showed the strongest connection to a subtype known as luminal A-like breast cancer.

Dr Luo, a recipient of the INSPIRE research grant, aimed at early career investigators, said: “We used large genetic datasets to examine morning preference, blood protein levels, and breast cancer risk. We also looked at different breast cancer subtypes and used breast cancer tissue data to assess the biological relevance of the proteins.”

Why does this matter?

World Cancer Research Fund International’s Assistant Director of Research and Policy, Dr Helen Croker, said:

“We know that there is a potential link between breast cancer risk and sleep chronotype, but this research has helped move the dial on our understanding of the biology behind this association.

The genetic tendency to be a morning person appears to be connected to a set of molecular switches, and some of those switches may also be linked to breast cancer risk. These findings improve our understanding of how sleep-related behaviour may influence breast cancer risk and further research could influence prevention strategies.”

Because chronotype is largely determined by our genes, this research is not a call to change your sleep habits. Its real potential lies in the proteins themselves: each one could be a possible new target for future cancer-prevention drugs or treatments, though that work is still at an early stage.

The researchers also note that their data came from people of European ancestry, so more research is needed to see whether the same proteins matter for people from other backgrounds.