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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