Cognitive performance may be enhanced or impaired depending on when it is measured, the type of cognitive task selected, and the type of exercise performed. (jensenlearning.com)
Research shows that time of day explains 20 percent of the variance on human performance on cognitive tasks. (scientificamerican.com)
But those things mainly affect speed: when healthy older people are given extra time to perform cognitive tasks, the results are on par with younger folks. (newscientist.com)