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Fact/Fiction: Does having a lie-in cause breast cancer?

Old news, truthfully retold. This week we delve into the facts behind the claim that women's body clocks affects their risk of breast cancer

How it was told

The link between sleep and cancer has been probed for years with a focus on whether 40 winks and a woman’s body clock affects the risk of breast cancer.

The latest headlines hit on November 6 after a team of University of Bristol researchers unveiled findings at the NCRI Cancer Conference in Glasgow that suggested night owls have a higher risk of developing the deadly disease. The story, claiming the risk of developing breast cancer drops by 48 per cent for those with a morning preference, was widely reported both in Britain and overseas – angled on both sides of the report.

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The Independent focused on the early risers with their article headlined “Waking up early ‘halves risk of developing breast cancer’”. The Scotsman followed suit with “Early rising ‘larks’ are less likely to develop breast cancer than night owls”, as did CNN.

The Daily Telegraph looked at it from the other perspective with their “Cancer risk for ‘night owls’ is twice as high as early risers” story. The Sun took a different approach entirely with a warning that “Women could halve their risk of breast cancer by going to bed early.”

Facts checked.

The research behind the story did not actually relate to when a woman gets out of bed – instead focusing on genetics and the preference for a time of day.

The investigation centred on the body clock, or the circadian rhythm as it is known by scientists, which is a24-hour cycle that affects sleepiness and alertness throughout the day.

These can vary per person, with larks preferring the morning and night owls who are more comfortable in the evening.

Dr Rebecca Richmond, one of the researchers on the project, said: “We would like to do further work to investigate the mechanisms underpinning these results, as the estimates obtained are based on questions related to morning or evening preference rather than actually whether people get up earlier or later in the day. In other words, it may not be the case that changing your habits changes your risk of breast cancer; it may be more complex than that.”

The data came from a questionnaire filled out by 180,215 women enrolled with the UK Biobank Project as well as 228,951 women who were part of the international Breast Cancer Association Consortium’s (BCAC) genetic data bank.

The women were asked whether they consider themselves to be a morning person or an evening person, as well as how many hours of sleep they got in 24 hours.

Researchers pored over the data using ‘Mendelian randomisation’, which uses genetic variants associated with possible risk factors, such as sleep characteristics, to investigate whether they contribute to breast cancer causes.

From there, they found that a preference for mornings reduced the risk of breast cancer by 40 per cent compared with being an evening type in the BCAC. It also uncovered that women who slept longer than the recommended seven to eight hours had a 20 per cent increased risk of the disease per additional hour slept.

As for the UK Biobank, morning preference reduced the risk of breast cancer by 48 per cent with one less person per 100 developing breast cancer if they are that way inclined.

It should also be noted that the study looked at genetics, so researchers took into account factors like family history. But genetic information does not necessarily correlate to reality, for example, a woman with a morning preference may work shifts that see her get up later.

So the true answer is much more nuanced and complex than being a night owl or a lark.

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Illustration: Miles Cole

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