Researchers have found that women who prefer to get up and go to be early have a 40 to 48 per cent lower risk of breast cancer than those whose body clock leads them to feel drowsy in the morning and most energetic in the evening. What's more, every additional hour slept after the recommended eight-hour sleep was associated with a 20 percent increase in risk.
In the Mendelian randomization analysis, they found approximately one less person per 100 will develop breast cancer if they have a morning preference compared with people who have an evening preference.
"So it may not be the case that changing your habits changes your risk of breast cancer - it may be more complex than that", she added.
Dr. Rebecca Richmond, a research fellow in the Cancer Research UK Integrative Cancer Epidemiology Program and the Epidemiology Unit at the University of Bristol, said in a presentation about the study reported by CNN: "We know that sleep is important generally for health".
The study did not prove a cause-and-effect relationship between sleeping patterns and breast cancer risk.
But they caution that it's too early to say whether being a night owl actually increases the risk of cancer or whether their preference for the evening is symptom of another, unknown risk factor.
That's according to European researchers looking at International Genetic Data.
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The data showed that women who were morning types, also known as "larks, ' had a reduced risk of breast cancer compared with those who were evening types, or 'owls".
She said policy-makers and employers should take note of the research.
Dipender Gill, of Imperial College London, said: "Although informative and interesting, this study alone does not warrant any action other than further investigation - people should not be changing their sleep patterns based on the evidence presented here".
Out of the 400 000 women, 2,740 were breast cancer survivors and 149 064 were disease free.
The Chief Executive Officer, AXA Mansard Health Limited, Mr Tope Adeniyi, said, "There is still no sure way to prevent breast cancer yet".
The NCRI Cancer Conference is taking place from 4-6 November 2018 at the Scottish Event Campus, Glasgow, UK.
NCRI works to coordinate research related to cancer, to improve the quality and relevance of research and to accelerate translation of research into clinical practice for the benefit of patients.
AXA Mansard, a member of AXA, a global leader in insurance and asset management, said it joined in the observance of the concluded breast cancer awareness.