Being a morning person is associated with a lower risk of developing breast cancer than being an evening person, a study* published by The BMJ has found.
Sleeping longer than the recommended 7-8 hours a night may also carry an increased risk of breast cancer, the results suggest.
Previous studies have shown a link between night shift work and risk of breast cancer, thought to be due to disrupted sleep patterns, light exposure at night, and other lifestyle factors. However, there has been much less research into the potential effects of sleep habits on breast cancer risk, so researchers set out to examine whether certain sleep traits could have a direct effect on risk of developing breast cancer.
Using a technique called Mendelian randomisation, they analysed genetic variants associated with three particular sleep traits - morning or evening preference (chronotype), sleep duration, and insomnia - for 180,216 women in the UK Biobank study and 228,951 women in the Breast Cancer Association Consortium (BCAC) study.
Analysing genetic information in this way avoids some of the problems that afflict traditional observational studies, making the results less prone to unmeasured (confounding) factors, and therefore more likely to be reliable. An association that is observed using Mendelian randomisation therefore strengthens the inference of a causal relationship.
In observational analysis of UK Biobank data, morning preference was associated with a slightly lower risk of breast cancer (one less woman per 100) than evening preference, whereas there was little evidence for an association with sleep duration and insomnia symptoms. However, the authors stress that this represents differences at the extreme ends of the scale, and that the extent of effect is likely to be smaller than that of other known risk factors for breast cancer, such as BMI and alcohol intake.
Mendelian analysis from BCAC also supported a protective effect of morning preference, and showed a potential harmful effect of longer sleep duration (more than the recommended 7-8 hours) on breast cancer, whereas evidence for insomnia symptoms was inconsistent.
The study relied on self-reported sleep measures and was restricted to women of European ancestry, so findings may not be applicable to other groups, but as several methods were used to assess data from two high-quality resources, the researchers say their findings “provide strong evidence for a causal effect of chronotype on breast cancer risk.”
Further work to uncover possible reasons for the associations between sleep disruption and breast cancer is required, they add. Nonetheless, these findings “have potential implications for influencing sleep habits of the general population in order to improve health.”
*Richmond RC, Anderson EL, Dashti HS, et al. Investigating causal relations between sleep traits and risk of breast cancer in women: mendelian randomisation study. BMJ 2019; 365 :l2327