Background/Objectives: The potential of outdoor physical activity as an intervention strategy to promote height growth velocity via stimulating growth hormone secretion and vitamin D synthesis has been scarcely investigated. The present study aimed to investigate the associations between outdoor physical activity duration and height growth velocity, and differences in gender, age, exposure time period (daily, school days vs. weekends), and body mass index (BMI) category. Methods: We performed a secondary analysis of longitudinal data from the 2019–2020 Chinese National Survey on Students’ Constitution and Health. The analytic sample included 5029 adolescents aged 9–18 years. High or low height growth velocity was defined as sex- and age-specific percentiles. Associations of high height growth velocity with outdoor activity duration (≥1 h, ≥2 h) on school days and weekends were investigated using multivariable logistic regression models. Analyses were stratified by sex, age group (9–12, 13–15, 16–18 years), and BMI category (normal weight, overweight, and obese). Results: Results from this cross-sectional analysis indicate that ≥1 h of daily outdoor physical activity is significantly associated with higher height growth velocity among normal-weight boys aged 9–15 years (OR range: 1.71–2.01) and girls aged 9–12 years (OR = 1.68). The positive association increased with ≥2 h (ORs up to 7.96). Consistently positive associations were found for activity during the school day compared to weekends. No significant associations were found in overweight and obese children. Conclusions: Ensuring adequate daily outdoor physical activity—especially on weekends—for at least two hours may be an important potential strategy to promote height growth in normal-weight children and adolescents. Interventions should consider differences in weight status and timing of activity.

Background/Objectives: The association between sleep deprivation and mental well-being in school-aged children and adolescents has been well documented, but the specific relationship between sleep debt and mental well-being remains unclear. This study aimed to evaluate the association between sleep debt and mental well-being in school-aged children and adolescents. Methods: This cross-sectional study included 4967 children and adolescents aged 9–18 years from eight provincial-level regions in China, using data from a multicenter school-based survey conducted within the framework of the Chinese National Survey on Students’ Constitution and Health (CNSSCH). Participants self-reported relevant sleep parameters, and the sleep debt was calculated based on the difference in sleep duration between weekdays and weekends. The main outcome was mental well-being. Adjusted covariates were age, sex, urban–rural residence, body mass index (BMI), vital capacity, and waist-to-hip ratio. Results: A total of 4967 children and adolescents were included (mean [SD] age 12.77 [2.59] years; 2475 [49.83%] girls) with 3176 participants classified as having higher mental well-being and 1791 participants classified as having lower mental well-being. After adjustment for covariates, sleep debt was associated with greater odds of lower mental well-being (adjusted odds ratio [aOR] = 1.095; 95% CI, 1.050–1.143; p < 0.001). Although statistically significant, the magnitude of this association was modest. In stratified analyses, each quartile increase in sleep debt was associated with 17.8% higher odds of lower mental well-being in boys (95% CI, 8.6–27.8%), 33.8% higher odds in girls (95% CI, 23.8–44.6%), 25.3% higher odds in urban students (95% CI, 16.3–35.0%), and 28.2% higher odds in rural students (95% CI, 17.6–39.7%) (all p < 0.001). In addition, weekend sleep patterns significantly interacted with the sleep debt–mental well-being association (p for interaction < 0.05), while age-specific nonlinear patterns involving nap duration were observed among adolescents aged 15–17 years (p for nonlinearity = 0.004). Conclusions: Sleep debt showed a statistically significant but modest cross-sectional association with lower mental well-being. Numerically larger subgroup-specific estimates were observed among girls and rural students. Given the cross-sectional design, these findings should not be interpreted as evidence of causality. Despite its modest magnitude, the association may have public health relevance at the population level. The observed patterns involving weekend sleep and age-specific nap duration warrant further evaluation in longitudinal and, where appropriate, intervention studies.