Social Patterning of 24-Hour Movement Behaviors Among Spanish Adolescents: Cross-Sectional Accelerometry Evidence from the EHDLA Study

Background/Objectives: The 24 h movement behavior (24HMB) framework integrates moderate-to-vigorous physical activity (MVPA), recreational screen time, and sleep. Sociodemographic correlates of simultaneous guideline adherence among European adolescents remain poorly understood. We aimed to estimate 24HMB compliance with each guideline and their combination, identify sociodemographic correlates, and compare accelerometry- and questionnaire-based sleep models. Methods: Cross-sectional data from 444 Spanish adolescents aged 12–17 years in the Eating Healthy and Daily Life Activities (EHDLA) study, conducted in a single semi-rural municipality in the Region of Murcia, were analyzed. MVPA and sleep were measured by ActiGraph wGT3X-BT accelerometry (valid days ranged from 1 to 6); sleep and screen time were also self-reported. Firth’s penalized logistic regression was applied uniformly across binary outcomes to provide a consistent estimation framework, with primary inference based on uncorrected p-values and Benjamini–Hochberg false discovery rate (FDR) correction reported for transparency. Results: In the subsample with complete screen-time data (n = 403; 51.6% female; median age 14 years), compliance was 26.1% (MVPA), 20.3% (screen time), and 36.7%/40.2% (accelerometry-derived sleep period time/questionnaire self-reported time in bed); only 3.5–4.0% met all three guidelines simultaneously. Female sex was associated with lower MVPA (odds ratio, OR = 0.51) and screen time (OR = 0.44) compliance (both FDR-corroborated). Caucasian ethnicity was unexpectedly associated with lower MVPA compliance (OR = 0.29) in the primary complete-case analysis. Older age was associated with lower screen-time compliance (OR = 0.45; both FDR-corroborated). No predictor was significant for accelerometry-derived sleep period time compliance, and questionnaire self-reported time in bed associations did not survive FDR correction. Conclusions: Concurrent adherence to 24HMB guidelines is critically low and socially patterned. Female sex was an FDR-corroborated correlate of lower MVPA and screen-time compliance. The Caucasian ethnicity association with MVPA was specific to the primary complete-case analysis and attenuated under stricter wear-time criteria. Sleep compliance estimates and their sociodemographic correlates differed by assessment method. Future 24HMB surveillance should use validated, standardized, and clearly reported protocols, and cross-study comparisons should account for methodological differences in sleep assessment.