Individual differences in the influence of time of day on executive functions.

Previous research has documented the impact of time of day on vigilance and memory and the moderating influence of circadian typology (morningness-eveningness) on the relationship between time of testing and cognitive processes. This study examined the impact of circadian typology and time of testing on executive functions. Morning-type and evening-type people were randomly assigned to a morning or afternoon testing time, and their performance on neuropsychological tests, including the Conners Continuous Performance Test (CPT), the Digit Span test, the Controlled Oral Word Association Test (COWAT), and the Wisconsin Card Sorting Test (WCST) was compared. No interactions between circadian typology and time of day were observed on the CPT or Digit Span test. A time of day effect was found on the letter generation portion of the COWAT. More words were generated in the afternoon than in the morning. In contrast, performance on the WCST was best when morning-type participants were tested at their optimal time of day. These findings suggest that circadian typology should be considered when scheduling neuropsychological assessments, rehabilitation training, and critical job tasks.