Can simply standing upright sharpen our minds? Whereas initial works proposed that standing posture reduces distractor interference in the Stroop task, subsequent replication attempts yielded mixed results. Importantly, previous studies mostly ignored individual fitness factors like BMI, physical activity level, and cardiac measures, which have recently been suggested to explain mixed findings. Here, we addressed these inconsistencies by controlling for such fitness characteristics in N = 36 healthy adults. Additionally, we expanded upon previous research by examining posture's influence on previously unexplored space-based cognitive control using the Navon task. We found evidence against an effect of standing posture on Stroop interference, while, in contrast, standing posture significantly improved spatial conflict processing in the Navon task. Most importantly, this effect was fully mediated by enhanced physiological arousal as indexed by reduced HRV, directly supporting the proposed mechanism of how standing influences cognitive control through increased postural demands. Our findings challenge the idea of a unified attentional control system, revealing that posture differentially influences conflict processing depending on its task-specific nature. These insights may advance theoretical understanding and have practical implications: in increasingly sedentary societies, simple changes like standing could selectively enhance certain cognitive functions, improving effective work and learning environments.