Sleep Duration and Cancer Risk: a Systematic Review and Meta-analysis of Prospective Studies

To assess the risk of cancers associated with sleep duration using meta-analysis of published cohort studies,we performed a comprehensive search using PubMed, Embase and Web of Science through October 2013.We combined hazard ratios (HRs) from individual studies using meta-analysis approaches. A random effectdose-response analysis was used to evaluate the relationship between sleep duration and cancer risk. Subgroupanalyses and sensitivity analyses were also performed. Publication bias was evaluated using Funnel plots andBegg’s test. A total of 13 cohorts from 12 studies were included in this meta-analysis, which included 723, 337participants with 15, 156 reported cancer outcomes during a follow-up period ranging from 7.5 to 22 years.The pooled adjusted HRs were 1.06 (95% CI: 0.92, 1.23; P for heterogeneity =0.003) for short sleep duration,0.91 (95% CI: 0.78, 1.07; P for heterogeneity <0.0001) for long sleep duration. In subgroup analyses stratifiedby cancer type, long duration of sleep showed an inverse relation with hormone-related cancer (HR=0.79; 95%CI: 0.65, 0.97; P for heterogeneity =0.009) and a greater risk of colorectal cancer (HR=1.29; 95% CI: 1.09, 1.52;P for heterogeneity =0.346). Further meta-analysis on dose-response relationships showed that the relative risksof cancer were 1.00 (95% CI: 0.99, 1.01; P for linear trend=0.9151) for one hour of sleep increment per day, and1.00 (95% CI: 0.98, 1.01; P for linear trend=0.7749) for one hour of sleep increment per night. No significantdose-response relationship between sleep duration and cancer was found on non-linearity testing (P=0.5053). Ourmeta-analysis suggests a positive association between long sleep duration and colorectal cancer, and an inverseassociation with incidence of hormone related cancers like those in the breast. Studies with larger sample size,longer follow-up times, more cancer types and detailed measure of sleep duration are warranted to confirmthese results.