Women who function better at the beginning of the day may have a reduced risk of developing breast cancer, the results of a large new study suggest.

UK researchers looked at over 409,000 women who had taken part in two major studies, one of which involves the largest collection of genetic data on women with breast cancer obtained to date.

They wanted to investigate if the way people sleep can contribute to the development of the disease.

An analysis of over 228,000 of the women found that those who are ‘larks', i.e. they prefer the mornings, had a 40% reduced risk of those who preferred the evenings (‘owls').

It also found that women who slept longer than the recommended seven to eight hours per night had a 20% increased risk of developing breast cancer per additional hour slept.

A further analysis involving over 150,000 women found similar results - larks had a 48% reduced risk of developing breast cancer.

"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. 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," commented Dr Rebecca Richmond of the University of Bristol.

The researchers believe these findings could have implications for both policy makers and employers.

"These findings have potential policy implications for influencing sleep habits of the general population in order to improve health and reduce risk of breast cancer among women," Dr Richmond added.

Details of these findings were presented at the 2018 UK National Cancer Research Institute (NCRI) Cancer Conference in Scotland recently.