A study published in The BMJ found that self-described early risers have a lower risk of breast cancer. “The findings showed consistent evidence for a protective effect of morning preference and suggestive evidence for an adverse effect of increased sleep duration on breast cancer risk,” the authors noted.
Researchers analyzed genetic variants associated with three sleep traits: sleep duration, insomnia, and morning or evening chronotype. They assessed data from the UK Biobank study (n=156,848; 7,784 with breast cancer) and Breast Cancer Association Consortium (BCAC) study (n=122,977 breast cancer cases and 105,974 controls).
Morning preference and breast cancer risk
In the UK Biobank study, early rising was inversely associated with breast cancer (hazard ratio = 0.95; 95% CI, 0.93-0.98); however, there was little evidence for an association between sleep duration and insomnia symptoms.
In the BCAC group, there was a protective effect of early rising (inverse variance weighted odds ratio [OR] = 0.88; 95% CI, 0.82-0.93) and an adverse effect of increased sleep duration (OR=1.19; 95% CI, 1.02-1.39 per hour increase) on breast cancer risk, while evidence for insomnia symptoms was inconsistent. Sleeping more than the recommended seven to eight hours a night was associated an increased risk—19% for every extra hour.
“One particular mechanism which might explain the link, known as the ‘light-at-night’ hypothesis, involves the suppression of melatonin levels in women exposed to artificial night at light, which in turn influences various hormonal pathways which might increase risk of breast cancer,” lead study author Rebecca Richmond, PhD, of the University of Bristol in the United Kingdom, told Reuters.