Women who are considered morning people are less likely to develop breast cancer than those who have more energy in the evenings, according to researchers.
The study, which compared data on hundreds of thousands of women, also found evidence of a causal link between sleeping longer and the disease.
The scientists found that those with an in-built morning preference were 40% to 48% less at risk of breast cancer.
Analysis showed women who slept longer than seven to eight hours a night – the amount recommended – increased their chances of being diagnosed with the disease by 20% per additional hour spent sleeping.
Researchers said people were genetically predisposed to being either “larks”, who tend to get up and go to bed early, or “owls”, whose body clocks tend to make them feel drowsy in the morning and energetic in the evening.
Lead scientist Dr Rebecca Richmond, of the University of Bristol, said researchers used genetic variants associated with people’s preference for morning or evening, sleep duration and insomnia before investigating whether they contributed to breast cancer developing.
“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,” she said.
“In other words, it may not be the case that changing your habits changes your risk of breast cancer; it may be more complex than that.
“However, the findings of a protective effect of morning preference on breast cancer risk in our study are consistent with previous research highlighting a role for night shift work and exposure to ‘light-at-night’ as risk factors for breast cancer.”
Those who took part in the study included more than 180,000 women in the UK Biobank project, which holds medical research data on 500,000 people.
Researchers also looked at results from almost 229,000 women signed up to an international genetic study carried out by the Breast Cancer Association Consortium.
Their findings were presented at the 2018 National Cancer Research Institute (NCRI) conference in Glasgow.
University of Manchester’s Cliona Clare Kirwan, a member of the NCRI Breast Clinical Studies Group, did not take part in the research but said it provided “further evidence of how our body clock and our natural sleep preference is implicated in the onset of breast cancer”.