Researchers have identified correlations between duration of sleep and cognitive decline with aging. There is at this point very little that can be said about mechanisms and the direction of causation, even speculatively, though the authors of this paper make an attempt at that:
Analysis of sleep and cognitive (brain function) data from 3,968 men and 4,821 women who took part in the English Longitudinal Study of Ageing (ELSA), was conducted. Respondents reported on the quality and quantity of sleep over the period of a month. In adults aged between 50 and 64 years of age, short sleep (less than 6hrs per night) and long sleep (more than 8hrs per night) were associated with lower brain function scores. By contrast, in older adults (65-89 years) lower brain function scores were only observed in long sleepers.
In this study, we did not find any significant interaction with gender. We found a significant decrease in amnestic and non-amnestic cognitive function in long sleepers, but this only reached significance in the older group. In our younger group, amnestic scores were significantly lower in short sleepers, whereas non-amnestic scores were lower in long sleepers. Our results show that an inverted U-shaped relationship exists in younger adults, where the amnestic scores for short sleepers were significantly lower than those for optimal sleepers, and whilst the amnestic scores for the long sleepers were reduced, this difference was not statistically significant. These findings could be interpreted in the context of recent findings in mice, which suggest that sleep deprivation causes irreversible damage to the brain which could impair cognitive function, particularly alertness. However, if this is the case, it is not clear why the effect of short sleep is not evident in the older group. That is, in the older adults, there was no observed effect in short sleepers but the amnestic scores in long sleepers were significantly lower than those for optimal sleepers.
In a study in men only, [previously published researcher] suggested that disturbed sleep is strongly associated with decline in executive function (or non-amnestic function), and less so for global cognition, whereas we found the opposite to be true in older adults. Indeed in our older group, the highest cognitive function scores (both amnestic and non-amnestic) were seen in those individuals with the greatest reported disturbances in sleep. In younger individuals however, there was no significant association between cognition and sleep quality, indicating that until we reach the age of around 65 years, there may be no association between sleep quality and cognitive function. The reason for these differences is unclear and prospective analyses of the effects of sleep quality on the decline in cognition could help rule out possible influences of reverse causality due to pre-existing ill-health or other confounders.
The suggestion that cognitive function increases with increasing sleep disturbance in older individuals appears to be counterintuitive. It may reflect the fact that those individuals who are more cognitively able are better at recording sleep disturbance data. Alternatively, it may indicate that in an elderly population, individuals who are more cognitively active may process the day's events and/or experience more worry or anxiety than those who are less cognitively active, and hence this may lead to an associated increase in self-reported frequency of sleep disturbance. Confounding effects of medications may also be more important in an older group. Likewise, in those participants with memory problems, we cannot exclude the possibility that their responses might have been erroneous to some extent due to their memory impairment.