The association of insomnia with subsequent breast cancer risk is largely unknown. Therefore, we assessed whether different symptoms of insomnia and their combination are associated with incident breast cancer in a large population-based study.Methods
In a prospective cohort study, 33,332 women were followed to monitor the occurrence of their first invasive breast cancer identified by the Cancer Registry of Norway. Insomnia symptoms including (1) nonrestorative sleep and (2) difficulty initiating and (3) maintaining sleep were self-reported using a study specific measure reflecting the current Diagnostic and Statistical Manual of Mental Disorders criteria. Hazard ratios and 95% confidence intervals were calculated using multiadjusted Cox proportional hazards models.Results
A total of 862 incident breast cancer cases occurred during a mean follow-up of 14.7 years. No consistent association was observed between the individual insomnia symptoms and breast cancer risk. However, compared to women reporting no insomnia complaints, those who reported having all three aspects of insomnia simultaneously were at increased risk (hazard ratio, 2.38; 95% confidence interval = 1.11–5.09).Conclusion
Our results suggest that having only some aspects of insomnia may not predispose someone to breast cancer. In contrast, experiencing all insomnia symptoms simultaneously might confer considerable excess risk.