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 response to exercise involving overload, potentially reflecting physiological adaptation. However, its response to continuous high-intensity exercise under controlled experimental conditions has not been sufficiently investigated. This pilot case series examined daily CAR changes during a 10-day high-intensity cycling protocol (20 min/day at 80% of maximal oxygen uptake [$\dot{V}{O}_2\max$]) in two healthy male participants. The CAR increased during Days 1–4 and returned to baseline levels from Day 5 onward, showing similar trends in acute physiological responses. $\dot{V}{O}_2\max$ and/or maximum workload improved following the intervention. These findings support the methodological feasibility of longitudinal CAR monitoring during short-term high-intensity exercise under controlled experimental conditions and suggest that CAR may be a promising non-invasive biomarker for assessing short-term physiological adaptation.