The researchers said they would like to perform further work because the estimates were based on questions related to morning or evening preference rather than actually whether people get up earlier or later in the day. People tend to be genetically programmed to fall into either of these categories.
Because these bits of DNA are set at birth and are not linked to other known causes of cancer, like obesity, it means the researchers are reasonably confident body clocks are involved in cancer.
"What we can be certain of is that all women - larks and owls - can reduce their risk of breast cancer by exercising regularly, maintaining a healthy weight and reducing their alcohol intake". And obesity is set to become the leading preventable cause of breast cancer for women in the United Kingdom, according to a report from earlier this year.
But if you are a morning person, feel free to give yourself yet another pat on the back.
'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.
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"The authors do not show any biological mechanism by which sleep timing preference could influence breast cancer risk".
The participants included 180,215 women enrolled with the UK Biobank project - a large long-term study which includes genomic data on about half a million people - and 228,951 women who had been part of a study on breast cancer carried out by the worldwide Breast Cancer Association Consortium (BCAC), which has the largest collection of genetic data on women with breast cancer in the world. "We would like to use genetic data from large populations to further understand how disrupting the body's natural body clock can contribute to breast cancer risk", she said.
Cliona Kirwan, consultant breast surgeon and researcher at the University of Manchester, said: "The use of Mendelian randomisation in this study enables the researchers to examine the causal effect on breast cancer of different sleep patterns".
"The statistical method used in this study, called Mendelian randomization, does not always allow causality to be inferred", said Dipender Gill, clinical research training fellow at Imperial College London.
However, cancer experts say modifying your sleep patterns probably won't have a significant impact on your cancer risk. The team presented their findings at the NCRI Cancer Conference in Glasgow, Scotland, while their paper, published on bioRxiv, awaits peer review.