A British researcher says women who have a body clock "in the morning" are less likely to develop breast cancer.
Researchers at the University of Bristol say the reason needs to be clarified.
The findings are important because they can affect the risk of all women.
Experts say the study published at Glasgow's NCRI Cancer Conference added to an understanding of the importance of sleep in all health conditions.
Everyone has a body clock that manages how the body works in a 24 hour pattern. It is also called a cycle rhythm.
It affects everything from sleep to even the risk of our mood and heart attack.
However, not everyone's clock is telling the same time.
People who live in the morning, or "camels", get up early, peak early, and get tired early in the evening.
Evening or "owls" tend to be more difficult to get up in the morning and prefer to get up later productively at night and sleep late.
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And is this related to breast cancer?
Researchers think so. They used a clever new method of data analysis called Mendelian randomisation.
They investigated 341 DNA snippets (guidelines for humans) that control whether we are likely to become species or owls.
They used this knowledge to conduct experiments on nearly 230,000 women in the British Breast Cancer Association Consortium study with more than 180,000 women in the British Biobank project.
People who were genetically programmed to have no chance of getting breast cancer were bigger than those who were programmed to be owls.
Because these DNA fragments are born at birth and not related to other causes known as cancer, such as obesity, researchers are convinced that the body clocks associated with cancer are reasonable.
How effective is it?
One in seven UK women will have breast cancer for life.
But this study saw a small snapshot of the life of an eight-year-old woman.
Two out of 100 ovaries with breast cancer at that time were compared to one out of 100 breast cancer-affected camels.
Rebecca Richmond, a researcher at the University of Bristol, told the BBC: "The results are very important because sleep is easily and easily transformed into ubiquitous.
"Previous studies have examined the impact of shift work, but this shows that it can 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 can be prevented.
So could a good night's sleep keep you from getting cancer?
It is not that simple.
Dr. Richmond said it is too early to give clear advice to women.
She told the BBC: "We must reach the evening man to make him more dangerous than the morning man, and we need to solve the relationship."
Is there anything about the body clock itself? Or is the "Owl" a victim to spend time in the body clock to get up and go to work? Does the body clock affect hormone levels, changing cancer risk, immune system or metabolism?
There are still many questions.
Is this a researcher?
Science can never be 100% sure, but it fits the emerging picture.
The World Health Organization (WHO) says it has already been linked to cancer risk if people's body clocks collapse because of shift work.
Dr. Richard Berks, of Breast Cancer Now, said, "This interesting result is evidence that there is some overlap between breast cancer and the risk of breast cancer when we prefer sleep, but more research is needed. Unravel the details of this relationship. "
A similar study found a role for the sleeping environment and mental health, including the risk of schizophrenia.
Cliona Kirwan, a consultant breast surgeon and researcher at the University of Manchester, said, "Using Mendelian randomisation in this study, researchers can investigate the causality of breast cancer in other sleep patterns.
"This is an interesting study that provides more evidence of how our body clocks and our natural sleeping environment are related to breast cancer outbreaks."
The results were published on the researcher's website, bioRxiv, but have not yet undergone a scientific peer review.
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