Aging and time-of-day effects on cognition in rats.

This study used an animal model to investigate the importance of the time at which testing occurs for age differences in learning and memory. Groups of old and young rats were entrained to a 12-hr light-dark schedule and administered tests of delayed alternation and inhibitory avoidance conditioning at the beginning or end of their high-activity cycle. Apart from normal age differences in test performance, the behavioral results demonstrated that old but not young rats were affected by the time of testing. In both tasks, old rats tested late in the activity cycle performed significantly worse than did old rats tested early in the cycle, under conditions that challenged memory processes that are known to involve the hippocampus. The results indicate that circadian disruption in old age can adversely affect memory and related cognitive function, with important implications for inhibitory control.