Effects of time of day on age-related differences in cognitive tests.

Previous studies have shown a shift in the circadian rhythm – and more particularly in the optimal time of day (OTD) – across the adult life span (May et al., 1993). The aim of this study was to investigate the relationship between cognitive efficiency and OTD in 113 healthy old adults (Age: M = 69, SD = 6.1, Range = 60-80) and 175 younger adults (M = 40.8, SD = 12.9, Range = 20-59). Participants performed a large battery of cognitive tests that assessed episodic memory, working memory, executive and attentional functions. Results on the MEQ (Morningness-Eveningness Questionnaire; Horne & Östberg, 1976) confirmed the age-related shift toward a self-reported morning preference in older adults. Second, the categorization of participants according to their MEQ scores and the time of testing revealed that the OTD has a greater impact upon cognitive performance in older than in younger adults. Third, the age-related OTD impact was more striking in working memory (Brown-Peterson and Pasat) and episodic memory tasks (Buschke) than in other aspects of the cognitive functioning. In conclusion, older participants tested during their peak circadian periods tend to show greater performance on memory tasks that require careful or strategic processing relative to older participants who are tested at off-peak times of day. Taken together, these findings indicate that care must be taken when investigators are considering the effects of age on effortful memory tasks, which are particularly modulated by OTD in older adults.