Women whose body clocks mean they are "morning people" have a lower risk of developing breast cancer, say researchers in the UK.
The University of Bristol team says the reason remains to be discovered.
He adds that the results are important because they can affect the risk of every woman.
Experts said the study presented at the NCRI Cancer Conference in Glasgow has increased understanding of the importance of sleep throughout health.
Everyone has a biological clock, which determines how the body works in a pattern of approximately 24 hours. It is also known as circadian rhythm.
It affects everything from the moment we sleep to our moods to the risk of a heart attack.
But not everyone's watch says the same time.
Morning people or "larks" are early to climb, peak early in the day and are tired in the early evening.
Night people or "owls" find it harder to get up in the morning, are productive later in the evening, and prefer to sleep late.
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And this is involved in breast cancer?
Researchers think so. They used a new and intelligent way of analyzing data – called Mendelian randomization.
They examined 341 DNA snippets (the instructions for the human body) that control whether we are likely to be a lark or an owl.
They used this knowledge to conduct an experiment on more than 180,000 women in the UK Biobank project and about 230,000 women in the Consortium study of the Breast Cancer Association.
They showed that people genetically programmed to be "larks" were less likely to get breast cancer than those programmed to be owls.
As these bits of DNA are defined at birth and are not linked to other known causes of cancer, such as obesity, this means that researchers are reasonably confident that body clocks are involved in cancer.
How big is the effect?
About one in seven women in the UK will develop breast cancer in their lifetime.
But this study looked at only a small snapshot of eight years of a woman's life.
At that time, two in 100 owls developed breast cancer compared to one in every 100 larks.
Rebecca Richmond, a researcher at the University of Bristol, told the BBC: "The findings are potentially very important because sleep is ubiquitous and easily modified.
"Previous research has looked at the impact of shift work, but this is showing that there may be a risk factor for all women."
Age and family history are some of the major risk factors for breast cancer. According to Cancer Research UK, about a quarter of cases may be preventable.
So a good night's sleep will stop me from having cancer?
It's not that simple.
Richmond said it was too early to give clear advice to women.
She told the BBC: "We still need to understand what makes a night person more at risk than a morning person … we need to disassociate the relationship."
Is it something about the biological clock itself? Or do the "owls" cause damage by living out of time with their body clocks to get up and go to work? Does the biological clock affect hormone levels to change the risk of cancer, immune system or metabolism?
There are still many unanswered questions.
Are the researchers right?
Science is never 100% sure, but it fits into an emerging picture.
The World Health Organization says that interrupting people's body clocks because of shift work is probably linked to the risk of cancer.
Dr. Richard Berks of Breast Cancer Now said: "These intriguing results increase the growing body of evidence that there is some overlap between genetics when we prefer to sleep and our risk of breast cancer but more research is needed to unravel the details of this relationship. "
Similar studies have revealed a role for sleep and mental health preferences, including the risk of schizophrenia.
Cliona Kirwan, a breast advisory surgeon and researcher at the University of Manchester, said: "The use of mendelian randomization in this study allows researchers to examine the causal effect on breast cancer of different sleep patterns.
"These are interesting findings that provide further evidence of how our biological clock and our natural preference for sleep are implicated in the onset of breast cancer."
The results were published on the researchers' bioRxiv website, but have not yet gone through scientific review.
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