Cognitive behavioral therapy for insomnia (CBT-I) is the preferred treatment for chronic insomnia and sleep-related cognitions are one target of treatment. There has been little systematic investigation of how sleep-related cognitions are being assessed in CBT-I trials and no meta-analysis of the impact of CBT-I on dysfunctional beliefs about sleep, a core cognitive component of treatment. Academic Search Complete, Medline, CINAHL and PsychInfo from 1990 to 2018 were searched to identify randomized controlled trials of CBT-I in adults (≥18 years) reporting some measure of sleep-related cognitions. Sixteen randomized controlled trials were identified comparing 1134 CBT-I and 830 control subjects. The Dysfunctional Beliefs and Attitudes about Sleep Scale was utilized almost exclusively to assess sleep-related cognitions in these trials. Hedge's g at 95% confidence interval (CI) was calculated to assess CBT-I effect size at post-treatment compared to controls. CBT-I significantly reduced dysfunctional beliefs about sleep (g = −0.90, 95% CI -1.19, −0.62) at post-treatment. Three trials contributed data to estimate effect size for long-term effects (g = −1.04, 95% CI -2.07, −0.02) with follow up time ranging from 3 to 18 mo. We concluded that cognitive behavioral therapy for insomnia has moderate to large effects on dysfunctional beliefs about sleep.