Cognitive performance varied across the day as a function of the synchrony between the time of testing and an individual's optimal period; best performance occurs at the time that coincides with optimal arousal. There are dramatic age differences in peak arousal: The majority of older adults are at their peak in the morning (when few college-attending younger adults are); many younger adults are at their peak late in the day (when virtually no older adults are). Thus, if experiments comparing younger and older adults are conducted in the afternoon, age differences will be exaggerated on tasks vulnerable to synchrony effects. The broad goals of the present project are to determine (1) the range of cognitive tasks and processes vulnerable to synchrony effects; (2) the degree to which there are age differences in the synchrony effect; and (3) the extent to which age differences have been exaggerated in the cognitive literature. Of central interest is the exploration of the behavioral and potential neurobiological mechanisms that are presumed to be particularly responsible for synchrony effects. Preliminary results strongly suggest that these mechanisms may be inhibitory in nature, and, as a result a general inhibitory theory of attention and its control over the contents of working memory is used to guide task selection. Because of recent suggestions implicating the role of the frontal lobes in aging and inhibitory control, the vulnerability of a number of neuropsychological tests to synchrony are explored. Three presumed functions of behavioral inhibition are tested: (1) access to working memory, (2) activation within working memory, and (3) preventing premature production of strong but unevaluated candidates for response. Consequences of diminished inhibition including heightened retrieval interference and increased reliance on heuristic decision rules will also be explored. The materials and tasks used are quite varied. These choices ensure (1) that a converging operations criterion will be used to assess the usefulness of the theoretical model and (2) that an extensive picture of the deficits of older adults at their nonoptimal times of day would emerge from the project. We explore the existence of circadian effects on frontal lobe functioning for both younger and older adults, in an attempt to link behavior and age differences to their underlying neurobiology. At a practical behavioral level, this work should reveal tasks that are relatively unimpaired by asynchrony and age as well as others which are severely impaired. Guidelines for maximizing older adults' performance on difficult tasks (e.g., making medical and legal decisions) are expected to emerge from this work.