Sleep characteristics, light at night and breast cancer risk in a prospective cohort

Increasing numbers of women in the US are getting too little sleep. Inadequate sleep has been associated with impaired metabolic function and endocrine disruption. Sister Study cohort participants (n = 50,884), completed baseline and follow‐up questionnaires on sleep patterns. Incident breast cancers estrogen receptor (ER) status of the tumor were ascertained from questionnaires and medical records. Cox regression was used to estimate adjusted hazard ratios (HRs) and 95% confidence intervals (95% CIs). Analyses of sleep characteristics reported at the first follow‐up interview included only participants who were breast cancer‐free at time of follow‐up interview. Over ∼7 years of follow‐up, 2,736 breast cancer cases (invasive and ductal carcinoma in situ) were diagnosed. There was little evidence that usual sleep duration or other sleep characteristics were associated with breast cancer. However, relative to those with no difficulty sleeping, women who reported having difficulty sleeping ≥ 4 nights a week were at an increased risk of overall (HR = 1.32, 95% CI: 1.09–1.61) and postmenopausal breast cancer (HR = 1.51, 95% CI 1.24–1.85). Risk of ER+ invasive cancer was elevated for women who reported having a light or television on in the room while sleeping (HR = 1.20, 95% CI: 0.97–1.47) or who typically got less sleep than they needed to feel their best (HR = 1.21, 95% CI: 0.98–1.50). In our study, most sleep characteristics, including sleep duration, were not associated with an increased risk although higher risk was observed for some markers of inadequate or poor quality sleep. What's new? This new study has uncovered a link between poor sleep and breast cancer risk. We already know that good quality sleep benefits health in many ways. The authors evaluated sleep habits and the association with breast cancer using detailed questionnaire data women provided when they enrolled in a cohort study. They determined that most sleep characteristics—including hours of sleep per night—did not correlate with cancer risk. However, women who reported difficulty falling asleep most nights, those with lights or TVs on in the room and those who felt they did not get enough sleep, showed a significant boost to their risk of breast cancer.