UK researchers say women whose body watches mean they're "morning people" has a lower risk of developing breast cancer.
The Bristol University team says the reason is still to be disclosed.
It adds that findings are important because they can affect the risk of each woman.
Experts said the study, presented at the NCRI Cancer Conference in Glasgow, augmented the growing awareness of the importance of sleep in all areas of health.
Everyone has a body clock that regulates body function in about 24 hours. It's also called the rhythm of the day.
It affects everything, from the time we sleep, with our mood and even on the risk of a heart attack.
But not every clock tells at the same time.
Morning people or "lark" are early to grow, peak early in the day and are tired earlier in the evening.
Evening people or "owls" are harder to get out of the morning, produce later in the evening and prefer to sleep late.
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Try our quiz to find out if you are a morning wearer or evening owl.
And is it related to breast cancer?
Researchers think so. They used a smart new way of analyzing data called Mendelian randomisation.
They looked at 341 DNA snippets (instructions for the human body) that control whether we are probably larva or owl.
They used this knowledge to experiment more than 180,000 women in the UK Biobank project and nearly 230,000 women's breast cancer consortium research.
They showed that those genetically engineered as "bears" had breast cancer less likely than those who were programmed as owls.
Since these DNA bits are born after birth and are not related to other known causes of cancer, such as obesity, it means that researchers are reasonably convinced that body wristbands are related to cancer.
How big is the impact?
About one in seven women in the UK will develop breast cancer throughout their lives.
But this study looked at only a small, eight-year-old snapshot of the woman's life.
At this time, it showed that two out of 100 owls had breast cancer compared to one of 100 lasers.
Dr. Rebecca Richmond, a researcher at the University of Bristol, told the BBC: "The conclusions are potentially very important because sleep is ubiquitous and easily modified.
"Previous studies looked at the effects of change work, but it shows that it can be a risk factor for all women."
Age and family history are some of the key risk factors for breast cancer. According to Cancer Research UK, about a quarter of the cases may be preventable.
So a good night's sleep will stop me from getting cancer?
It is not so simple.
Dr. Richmond said that it was said that it was still too fast to give women clear advice.
She told the BBC: "We still need to get what makes the evening man more at risk than a man of the morning … we have to find out the relationship."
Is it something about a body clock in itself? Or "owls" can cause damage by living over time with body clocks to get up and go to work? Does your body clock affect the hormone level to change the risk of cancer, the immune system or the metabolism?
There are still many unanswered questions.
Are scientists right?
Science has never been 100% sure, but it matches the latest image.
The World Health Organization has already said that stopping a human body clock due to work shift may be due to cancer risk.
Dr. Richard Berk of Breast Cancer has now said: "These intriguing results add to the growing evidence that genetics exists when we sleep and our risk of breast cancer is overlap, but more research is needed to read the peculiarities of this relationship."
Similar studies have identified the importance of sleep choices and mental health, including the risk of schizophrenia.
Clinic Kirwan, a breast surgeon and researcher at the University of Manchester, said: "Using Mendel's tests in this study allows researchers to explore the causal relationship between various sleep patterns on breast cancer.
"These are interesting conclusions that provide further evidence of how our body clock and our natural sleep choices are associated with the emergence of breast cancer."
Opinions have been published on the bioRxiv researcher site, but have not yet made a scientific comparison.
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