Chronobiological effects of morning and evening exercise on biochemical responses in elite boxers

This study aimed to investigate the chronobiological effects of exercise timing, specifically morning versus evening sessions, on biochemical responses in elite boxers. While both high-intensity interval training (HIIT) and moderate-intensity continuous training (MICT) protocols were applied, the primary focus was to examine how the time-of-day influences markers of muscle damage, oxidative stress, and hormonal activity. Twenty-one elite male boxers completed four exercise sessions, two HIIT and two MICT, conducted in the morning and evening on separate days. Venous blood samples were collected immediately after each exercise session and again one-hour post-exercise. A 3 × 2 × 2 mixed-design ANOVA with repeated measures was used to analyze the effects of exercise type, time of day, and sampling time on biochemical markers. Significant differences were observed in all muscle and cardiac output biomarkers (LDH, CK, CK-MB, TnT, MB) across sampling times (rest, immediately post-exercise, and one-hour post-exercise). Additionally, CK levels showed a significant difference based on the time of day, with higher values recorded in evening sessions. Regarding oxidative stress parameters, a significant three-way interaction (exercise type × time of day × sampling time) was observed for malondialdehyde (MDA), a serum oxidant marker. IL-6 levels differed significantly by both time of day, with higher levels observed in morning sessions and sampling time, with elevated concentrations immediately after and 1 hour after exercise compared to rest. A significant interaction between time of day and sampling time was also detected for IL-6. Based on the findings of this study, both HIIT and MICT appear to induce more favorable biochemical responses when performed in the morning. Morning exercise sessions were associated with reduced markers of muscle damage, oxidative stress, and pro-inflammatory activity compared to evening sessions. Therefore, morning exercise may be recommended to optimize recovery, minimize physiological strain, and support overall athletic performance in elite athletes.