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Winocur, G., & Hasher, L. (2002). Circadian rhythms and memory in aged humans and animals. In L. Squire and D. Schacter (Eds.), Neuropsychology of Memory, 3rd Edition (pp. 273-285). New York: Guilford Publishers.


(Excerpt from Introduction) People have strong preferences for the times at which they engage in everyday activities, such as shopping, reading newspapers, and listening to music (Yoon, 1998). These preference patterns are quite different across the adult lifespan, with college-age young adults preferring to perform many activities in the afternoon and evening, and older adults preferring the morning (Hasher, Zacks, & May, 1999; May, Hasher & Stoltzfus, 1993). One explanation for these patterns is that by the afternoon, older adults are tired and lack motivation, whereas young people are more likely to be alert and functioning at near-optimal levels. In fact, the scenario may be more complicated than this; it may relate to diurnal biological rhythms that are different for older and younger adults, and that in addition are disrupted in old age. The altered behavioral patterns are consistent with a substantial shift toward peak arousal and activity in the morning that has been found for both older humans and lower animals (see, Hoch et al., 1992; Ingram, London & Reynolds, 1982; Peng & Kang, 1984). This age-related shift has been linked to changes in a wide range of circadian rhythms that affect, for example, sleep-wake cycles, eating and drinking patterns, glucose uptake, and heart rate, as well as circulating hormones (e.g., melatonin, adrenocorticotropic hormone) and neurotransmitter function (e.g. acetylcholine, norepinephrine; Brock, 1991; Burwell, Whealin, & Gallagher, 1994; Edgar, 1994; Horne & Ostberg, 1977; Hrushesky, 1994; Ingram et al., 1982; Stone, 1989).

Recent work in several laboratories suggests that variation in circadian arousal rhythms may also influence human cognitive function, with optimal performance associated with testing that occurs at near peak times of arousal as opposed to off-peak times. This pattern is termed the "synchrony effect" (May & Hasher, 1998). Given considerable evidence that older and younger adults experience peak arousal times at different times of the day (e.g. Adan & Almirall, 1990; May & Hasher, 1998; Mecacci & Zani, 1983; Yoon, 1998), the suggestion that age-related patterns of performance could well differ across the day, from morning (a peak time for as many as 75% of older adults) to late afternoon (near a peak time for at least 35% of college students).

From a neuropsychological perspective, an interesting feature of the various inhibitory tasks is that they are associated with frontal lobe function. Indeed, it has been suggested that the synchrony effect in aged humans is the direct result of circadian disruption of frontal lobe function (Intons-Peterson et al., 1999; May & Hasher, 1998).

The question arises as to whether a synchrony-like effect can be demonstrated in an animal model. Although research in this area is in its infancy, this effect does appear to occur. Work in our laboratory and others (e.g. Gallagher & Burwell, 1989; Stone, 1989; Winocur & Hasher, 1999) has shown that age differences on tests of learning and memory in rats are greater late in the animals' activity cycle, when arousal levels are lowest, than at the beginning of the cycle. Of particular interest, this pattern of performance coincides with significant alterations in measures of circadian rhythmicity in the older rats.

In this chapter, we review recent and current work that demonstrates a link between altered circadian patterns in old age and cognitive performance, with a particular emphasis on memory function. Two research programs, involving aged humans and animals, are described; although the respective paradigms and strategies necessarily differ, there is a remarkable convergence in the findings. The results of this research highlight the potential importance of disrupted circadian rhythms for cognitive aging, as well as providing some insights into their neuropsychological correlates.

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