Women whose body clocks are "morning people" have a lower risk of developing breast cancer, British researchers say.
The University of Bristol team explains why it still needs to be discovered.
The results are important because they can affect the risk for every woman.
Experts said that the study presented at the NCRI Cancer Conference in Glasgow contributed to a growing understanding of the importance of sleep in all diseases.
Everyone has a body clock that determines the way the body works in about 24 hours. It is also known as circadian rhythm.
It affects everything when we sleep, our mood and even our heart attack risk.
But not all clock shows the same time.
Morning people or "Larks" rise early, reach the day earlier and are tired in the evening.
Evening people or "owls" have difficulty getting up in the morning, are productive later in the evening, and prefer to sleep late.
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Take our quiz to find out if you are an owl in the morning or in the evening.
And that's involved in breast cancer?
The researchers think so. They used a clever new method for data analysis – the Mendelian randomization.
They examined 341 DNA snippets (the instructions for the human body) that control whether we're probably a lark or an owl.
They used this knowledge to conduct an experiment involving more than 180,000 women in the British Biobank project and nearly 230,000 women in the study of the Breast Cancer Association consortium.
They showed that people genetically programmed as "larks" were less likely to suffer from breast cancer than those programmed as owls.
Because these pieces of DNA start at birth and are not linked to other known causes of cancer such as obesity, it means researchers are pretty sure that the body clocks are involved in cancer.
How big is the effect?
Around one in seven women in the UK suffer from breast cancer during their lifetime.
In this study, however, only a small, eight-year snapshot of a woman's life was considered.
During this time, two of 100 owls showed breast cancer compared to one in 100 larks.
Dr. Rebecca Richmond, one of the researchers at the University of Bristol, told the BBC: "The results may be very important because sleep is ubiquitous and can be easily modified.
"Previous studies looked at the effects of shiftwork, but this shows that there may be a risk factor for all women."
Age and family history are some of the major risk factors for breast cancer. About a quarter of the cases could be avoidable, according to Cancer Research UK.
So, will a good sleep prevent me from getting cancer?
It is not so easy.
Dr. Richmond said it was too early to give women clear advice.
She told the BBC, "We still have to figure out what makes an evening person more vulnerable than a morning person … we need to break the relationship."
Is it something with the body clock? Or do "owls" harm by living out of time with their body clock to get up and go to work? Does the body clock affect hormone levels to alter the risk of cancer or the immune system or metabolism?
There are still many unanswered questions.
Are the researchers right?
Science is never 100% sure, but that suits an emerging picture.
The World Health Organization (WHO) already says that interrupting people's body clock through shift work is likely related to cancer risk.
Dr. Breast Cancer Now's Richard Berks said, "These intriguing results add to the growing body of evidence that genetics overlap when we'd rather sleep and our risk of breast cancer, but more research is needed. Unravel the peculiarities of this relationship."
Similar studies have shown that sleep preferences and mental health play a role, including the risk of schizophrenia.
Cliona Kirwan, consultant breast surgeon and researcher at the University of Manchester, said, "The use of Mendelian Randomization in this study allows researchers to investigate the causal effects of different sleep patterns on breast cancer.
"These are interesting findings that provide further evidence of how our body clock and our natural preference for sleep are associated with the onset of breast cancer."
The results were published on researchers' bioRxiv website, but have not yet been peer-reviewed.
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