OBJECTIVES: The aim of this study was to test whether a cognitive behavioral and mindfulness-based group sleep intervention would improve sleep and anxiety on school nights in a sample of at-risk adolescents. We also examined whether benefits to sleep and anxiety would be mediated by improvements in sleep hygiene awareness and presleep hyperarousal. METHODS: Secondary analysis of a randomized controlled trial conducted with 123 adolescent participants (female = 60%; mean age = 14.48) who had high levels of sleep problems and anxiety symptoms. Participants were randomized into a sleep improvement intervention (n = 63) or active control "study skills" intervention (n = 60). Preintervention and postintervention, participants completed the Pittsburgh Sleep Quality Index (PSQI), Spence Children's Anxiety Scale (SCAS), Sleep Beliefs Scale (SBS), and Presleep Hyperarousal Scale (PSAS) and wore an actiwatch and completed a sleep diary for five school nights. RESULTS: The sleep intervention condition was associated with significantly greater improvements in actigraphy-measured sleep onset latency (SOLobj), sleep diary measured sleep efficiency (SEsubj), PSQI, SCAS, SBS, and PSAS, with medium to large effect sizes. Improvements in the PSQI and SCAS were specifically mediated by the measured improvements in the PSAS that resulted from the intervention. Improvements in SOLobj and SEsubj were not specifically related to improvements in any of the putative treatment mechanisms. CONCLUSIONS: This study provides evidence that presleep arousal but not sleep hygiene awareness is important for adolescents' perceived sleep quality and could be a target for new treatments of adolescent sleep problems.