Changes in Cortisol Awakening Response During 10 Days of High-Intensity Cycling Exercise

Previous studies suggest that the cortisol awakening response (CAR) shows a biphasic pattern—either an increase or a blunting—in reaction to exercise involving overload. These changes are considered to reflect physiological adaptation to exercise. However, the effects of continuous exercise on CAR under controlled experimental conditions have not been sufficiently explored. This study examined changes in CAR in response to short-term, continuous high-intensity cycling exercise over a 10-day period. Two healthy male participants underwent 12 consecutive days of experimental sessions: 2 baseline rest sessions followed by 10 days of cycling exercise sessions (20 min/day at 80% of maximal oxygen uptake [VO₂max]). CAR was measured daily throughout this period. Acute physiological responses and aerobic capacity were also assessed. Results showed that CAR increased during Days 1–4 of the exercise period and returned to baseline levels from Day 5 onward. Similar trends were observed in physiological responses. Additionally, participants demonstrated improvements in VO2max and/or maximum workload following the experimental sessions. These findings suggest that CAR could be a useful non-invasive biomarker for monitoring short-term physiological adaptation to high-intensity exercise.