Women whose body clocks mean they are "morning people" have a lower risk of developing breast cancer, say UK researchers.
The team at the University of Bristol says the reason why still needs to be uncovered.
It adds the findings are important as they may affect every woman's risk.
Experts said the study presented at the NCRI Cancer Conference in Glasgow added to a growing understanding of the importance of sleep in all health.
Everybody has a body clock, which governs how the body works in a roughly 24-hour pattern. It's also known as a circadian rhythm.
It affects everything from when we sleep, to our mood and even our risk of a heart attack.
But not everybody's clock tells the same time.
Morning people or "larks" are early to rise, peak earlier in the day and are tired earlier in the evening.
Evening people or "owls" find it harder to get up in the morning, are productive later into the evening and prefer to go to sleep late.
Take our quiz to find out whether you are a morning type, or an evening owl.
And this is involved in breast cancer?
The researchers think so. They used a clever new way of analysing data - called Mendelian randomisation.
They looked at 341 snippets of DNA (the instructions for the human body) that control whether we are likely to be a lark or an owl.
They used this knowledge to perform an experiment on more than 180,000 women in the UK Biobank project and nearly 230,000 women in the Breast Cancer Association Consortium study.
They showed people genetically programmed to be "larks" were less likely to have breast cancer than those programmed to be owls.
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.
How big is the effect?
Around one in seven women in the UK will develop breast cancer in their lifetime.
But this study only looked at a small, eight-year snapshot of a woman's life.
In that time it showed two in 100 owls developed breast cancer compared with one in 100 larks.
Dr Rebecca Richmond, one of the researchers from 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 there may be a risk factor for all women."
Age and family history are some of the main risk factors for breast cancer. According to Cancer Research UK, around a quarter of cases might be preventable.
So will a good night's sleep stop me getting cancer?
It's not that simple.
Dr Richmond said it was said it was still too soon to give clear advice to women.
She told the BBC: "We still need to get at what makes an evening person more at risk than a morning person… we need to unpick the relationship."
Is it something about the body clock itself? Or do "owls" cause damage by living out of time with their body clocks in order to get up and go to work? Does the body clock affect hormone levels to alter cancer risk, or the immune system, or metabolism?
There are still many unanswered questions.
Are the researchers right?
Science is never 100% sure, but this fits with an emerging picture.
The World Health Organization already says disruption to people's body clocks because of shift work is probably linked to cancer risk.
Dr Richard Berks, from Breast Cancer Now, said: "These intriguing results add to the growing body of evidence that there is some overlap between the genetics of when we'd prefer to sleep and our breast cancer risk, but more research is required to unravel the specifics of this relationship."
Similar studies have revealed a role for sleep preferences and mental health, including schizophrenia risk.
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.
"These are interesting findings that provide further evidence of how our body clock and our natural sleep preference is implicated in the onset of breast cancer."
The findings have been published on researchers' website bioRxiv but have not yet gone through scientific peer review.
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