Morning people less likely to develop breast cancer
Being a “morning person” appears to lower the risk of breast cancer in women, while sleeping for more than the recommended 7–8 hours per night may increase such risk, suggests a study.
“[There is] strong evidence for a causal effect of chronotype on breast cancer risk,” said researchers. “Furthermore, some evidence suggested a causal effect of sleep duration on risk of breast cancer, although findings for these traits were less consistent across the different methods applied.”
Morning preference showed an inverse relationship with breast cancer (hazard ratio [HR], 0.95, 95 percent CI, 0.93–0.98 per category increase), while little evidence indicated that sleep duration was associated with insomnia symptoms in multivariable regression analysis using data from the UK Biobank. [BMJ 2019;365:l2327]
A total of 341 single nucleotide polymorphisms (SNPs) associated with chronotype, 91 SNPs linked to sleep duration and 57 SNPs related to insomnia symptoms were used. In one sample Mendelian randomization (MR) analysis using data from the UK Biobank, there was supportive evidence for a protective effect of morning preference on breast cancer risk (HR, 0.85, 0.70–1.03 per category increase), but estimates for sleep duration and insomnia symptoms were loose.
Two sample MR in the Breast Cancer Association Consortium (BCAC) study also found a protective effect of morning preference (inverse variance weighted odds ratio [OR], 0.88, 0.82–0.93 per category increase) and an adverse effect of longer sleep duration (OR, 1.19, 1.02–1.39 per hour increase) on breast cancer risk (both oestrogen receptor positive and negative types). On the other hand, there was inconsistent evidence for insomnia symptoms.
Overall, results were robust to sensitivity analyses accounting for horizontal pleiotropy.
“Findings of an adverse effect of evening preference on breast cancer risk in all analyses performed go some way to supporting hypotheses around carcinogenic light-at-night and findings of increased risk among night shift workers who might be exposed to artificial light at night,” researchers said. [Lancet Oncol 2007;8:1065-1066; Int J Epidemiol 2009;38:963-970]
Particularly, the specificity of the causal effect of chronotype on breast cancer, which was not seen for other cancers or all-cause mortality, support the hormonal mechanisms implicated in the light-at-night hypothesis, they added. When using an objective chronotype measure (ie, the least active 5 hours [L5]), however, the findings did not show the same adverse effect.
This MR analysis used data from the UK Biobank prospective cohort study (n=156,848 women; n=7,784 with breast cancer) and the BCAC case-control genome-wide association study (n=122,977 breast cancer cases; n=105,974 controls). Exposures included self-reported chronotype (morning or evening preference), insomnia symptoms and sleep duration in multivariable regression, and genetic variants associated with these sleep traits.
Further research is warranted to determine whether it is the actual behaviour or the preference that poses the health risk, to examine the impact of circadian misalignment—which can be determined by genetic risk, self-reported chronotype and objectively measured L5 timing—and to investigate the suggestive evidence for a causal effect of longer sleep duration on breast cancer risk.