New large-scale research has found that women who are naturally morning people may have a lower risk of breast cancer than those who are night owls.
Carried out by UK, US, French, and Norwegian researchers, the new study looked at or 180,216 women taking part in the UK Biobank study, a large long-term study which includes genomic data on more than half a million UK residents, and 228,951 women in the Breast Cancer Association Consortium (BCAC) study.
Using a technique called Mendelian randomization, which involves studying genetic variants to see if certain factors are associated with a higher or lower risk of disease, the researchers analysed the women's genetic variants associated with three particular sleep traits -- a morning or evening preference (known as chronotype), sleep duration, and insomnia.
As Mendelian randomization gives more reliable results, if an association is found it is more likely to suggest a direct relationship.
The findings, published by The BMJ Thursday, showed consistent evidence that being a morning person was associated with a lower risk of developing breast cancer than being an evening person.
There was also suggestive evidence that sleeping longer than the recommended 7 to 8 hours a night was associated with an increased risk of the disease, although the findings were unclear for what effect insomnia may have on breast cancer risk.
The team added that the effects of sleep habits are likely to be smaller than the effects of other known risk factors for breast cancer, such as Body Mass Index (BMI) and alcohol intake, although they add that the findings do have implications for influencing sleep habits to improve health.
They also noted that the study does have some limitations, for example it used self-reported sleep measures which can be subject to inaccuracies, and looked only at women of European ancestry, so findings may not be applicable to other groups. However, a strength of the study is that the researchers used several methods to assess data from two high quality studies.
Women who do night shift work have also been found to have an increased risk of breast cancer, possibly due to this type of work disrupting sleep patterns and exposing women to light during the night time.