Timing is everything: Age differences in the cognitive control network are modulated by time of day.

Behavioral evidence suggests that the attention-based ability to regulate distraction varies across the day in synchrony with a circadian arousal rhythm that changes across the life span. Using functional magnetic resonance imaging (fMRI), we assessed whether neural activity in an attention control network also varies across the day and with behavioral markers. We tested older adults in the morning or afternoon and younger adults tested in the afternoon using a 1-back task with superimposed distractors, followed by an implicit test for the distractors. Behavioral results replicated earlier findings with older adults tested in the morning better able to ignore distraction than those tested in the afternoon. Imaging results showed that time of testing modulates task-related fMRI signals in older adults and that age differences were reduced when older adults are tested at peak times of day. In particular, older adults tested in the morning activated similar cognitive control regions to those activated by young adults (rostral prefrontal and superior parietal cortex), whereas older adults tested in the afternoon were reliably different; furthermore, the degree to which participants were able to activate the control regions listed above correlated with the ability to suppress distracting information.