An inverted U-shaped relationship between cortisol awakening response and same-day error monitoring function in healthy males

•CAR is thought to provide an energetic “boost” for the coming day.•No association was observed between CAR and post-error behavior.•A significant quadratic relationship was observed between CAR and same-day ERN amplitude.•Moderate CAR was associated with best error monitoring function in healthy males.•Cortisol awakening response could be a biological predictor of cognitive function in the same day. The cortisol awakening response (CAR) is thought to provide an energetic “boost” for the coming day and has been shown to be associated with prefrontal dependent function. The aim of the current study was to examine the relationship between the CAR and same-day neural activity following an error response task. Forty-six healthy males (22.25 years ± 1.98) collected saliva samples at 0, 30 and 45 min post-awakening to measure the CAR, and on the same afternoon event-related potential activity during error processing was measured. Although no association was observed between CAR and post-error behavior, a significant quadratic relationship was observed between CAR and Error-related Negativity (ERN) amplitude, and this association remained while controlling for confounding factors. This finding suggested the existence of an inverted U-shaped relationship between CAR and the same-day error-monitoring function in healthy males.