Effects of morning versus evening high-intensity exercise on electroencephalographic and autonomic responses in middle-aged women

[Purpose] The time of day at which exercise is performed influences physiological responses and performance, and thus its health benefits. This study investigated whether electroencephalographic (EEG) and autonomic nervous system responses to morning versus evening exercise differed in middle-aged women.[Methods] Thirty women aged 50-60 years were randomly assigned to a morning or evening exercise group. Participants were asked to sleep for seven to eight hours before testing. The morning group exercised at 6:00 am and the evening group at 6:00 pm. EEG-derived indices included the theta-to-beta ratio (TBR) and spectral edge frequency at 90% (SEF-90). Photoplethysmography-derived autonomic indices comprised the HRV index and HRV-derived sympathetic-related and parasympathetic-related indices, measured at rest before and immediately after exercise. Data were analyzed using a two-way mixed-design analysis of variance. For significant interactions, simple effects and change-score analyses were performed.[Results] TBR and SEF-90 showed significant group × time interactions, with greater changes from pre- to post-exercise in the morning group. TBR increased and SEF-90 decreased following exercise in both groups, with greater changes in the morning group. The HRV index showed only a significant main effect of time. However, HRV-derived sympathetic-related and parasympathetic-related indices demonstrated significant group × time interactions, with greater autonomic responses in the morning group, as measured by change scores.[Conclusion] EEG- and HRV-derived neurophysiological response patterns to high-intensity exercise may vary with exercise timing. Morning high-intensity exercise may elicit greater neurophysiological responses than evening exercise, highlighting the importance of exercise timing in training strategies and recovery.