The current study tested the developmental significance of both early adolescent sleep quantity and quality for academic competence and internalizing and externalizing problems over the course of 2 years. As part of an accelerated longitudinal study, data were collected from N = 586 Czech adolescents (Mage = 12.34 years, SD = .89, 58.4% female). Data analyses included a series of logistic regressions that controlled for adolescent sex, age, family structure, and socioeconomic status. Findings showed that sleep quality at Wave 1 predicted developmental changes 1 year later (Wave 3) in depression, anxiety, and low self-esteem (ORrange = 1.7-1.8) and 2 years later (Wave 5) in externalizing behaviors (OR = 2.6). Importantly, despite the associations observed with Wave 3 anxiety and deviance, Wave 1 sleep quantity was unrelated to subsequent developmental changes in adjustment measures, both 1 and 2 years later. No sleep effects at all were observed on a variety of measures of academic competence. Study findings underscore the developmental significance of sleep and indicate greater salience of sleep quality vis-à-vis sleep quantity. They also replicate some of the observed relationships found in previous longitudinal work on the sleep-mood link but extend the sleep-adolescent adjustment literature in a number of important ways. (PsycInfo Database Record (c) 2021 APA, all rights reserved).