BACKGROUND: Insomnia is an important but often overlooked side effect of cancer. Dysfunctional sleep beliefs have been identified as an important perpetuating factor for insomnia. Mindfulness practice has been demonstrated to improve sleep quality but it is unknown whether these effects relate to changes in dysfunctional sleep beliefs. PURPOSE: This study is a secondary analysis of a randomized controlled trial comparing mindfulness-based cancer recovery (MBCR) to cognitive behavior therapy for insomnia (CBT-I) in cancer patients with insomnia. This present analysis compares program impact on mindfulness, dysfunctional sleep beliefs, and insomnia severity clinical cutoffs. METHODS: Patients (MBCR, n = 32; CBT-I, n = 40) were assessed at baseline, post-program, and 3-month follow-up. RESULTS: Across both groups, patients showed improvements over time in acting with awareness (P = .021) and not judging experiences (P = .023). Changes in dysfunctional sleep beliefs produced by the CBT-I group exceeded those produced by MBCR at post-program and follow-up (P < .001). Acting with awareness, non-judging, and non-reacting were the facets of mindfulness associated with an overall reduction in dysfunctional sleep beliefs. There were no significant differences between the MBCR and CBT-I groups in the percentage of patients exceeding insomnia severity clinical cutoffs at post-program or follow-up. CONCLUSIONS: This study supports the use of both CBT-I and MBCR to reduce insomnia severity and suggests the development of mindfulness facets as a method of reducing dysfunctional sleep beliefs.