Exercise Timing in Sport: Molecular and Physiological Mechanisms Linking Performance, Recovery, and Biological Cost

Time of day is often treated as a scheduling constraint rather than a true programming variable, yet training time is not neutral and can modulate both performance expression and the biological cost of exercise. This structured narrative review synthesizes evidence on morning versus afternoon/evening training in athletes and physically active adults, integrating performance outcomes with internal load, autonomic recovery, sleep interaction, and downstream physiological stress. Acute evidence generally supports superior late-afternoon or early-evening performance, particularly for neuromuscular tasks, whereas chronic training studies do not indicate a universal optimal training time. Instead, adaptation appears to depend on chronotype, habitual training time, the wake-to-training interval, light exposure, nutritional timing, training-testing congruency, and competition-specific demands. Mechanistically, exercise timing likely interacts with BMAL1/CRY loops, AMPK-SIRT1-PGC-1alpha signaling, PGC-1alpha-driven mitochondrial remodeling, and redox-inflammatory pathways, although much of this evidence remains preclinical. Accordingly, this review advances a Performance-Biological Cost (PBC) framework in which training time is judged by the balance between output, recovery opportunity, and residual molecular cost rather than by acute performance alone. By integrating athlete-relevant human evidence with mechanistic insights and translating them into explicit decision tools and testable predictions, this review positions exercise timing as a context-dependent lever within periodized training design.