Body and Mind Intertwined: Physical Activity Intensity, Sleep Quality, and Depressive Symptoms: Cross-Sectional Associations and Exploratory Path Analysis

Background: The alarming trend of rising depression and insomnia among the world’s population, especially in young adults, demands urgent attention from both clinical and lifestyle medicine perspectives. This study aimed to evaluate whether habitual physical activity (PA) could improve sleep quality and depressive symptoms. It explores whether sleep parameters mediate the relationship between training intensity and depression. Methods: This cross-sectional study examined the relationships between PA, sleep quality, and depression in 349 participants, predominantly young adults, aged 16–54 (Median: 21, Q1–Q3: 20–23). Depressive symptoms and sleep quality were assessed using the Beck Depression Inventory (BDI) and Pittsburgh Sleep Quality Index (PSQI). PA parameters, including training intensity and frequency, were evaluated via a study-specific self-report tool. Mediation effects were assessed utilising Structural Equation Modelling (SEM) to evaluate direct, indirect, and total effects. Results: Higher training intensity and training volume were associated with lower depressive symptoms. Except for the daytime dysfunction domain, the current data did not support sleep as a mediator in the relationship between physical activity and depressive symptoms. Participants with 3–5 years of training experience exhibited lower BDI scores compared to those with less than 12 months of experience. Interestingly, the type of PA did not influence BDI or PSQI scores, suggesting that consistency and intensity are more critical than the kind of exercise itself. Conclusions: Systematic, high-intensity PA, especially for a prolonged period of time, is significantly associated with lower depressive symptoms. The exploratory nature of this study highlights training frequency and intensity as potential factors that need further research. These findings highlight the mind–body connection and demonstrate that PA might be an intervention supporting mental well-being.