Sleep, Circadian Behaviour, Physical Activity, Social Networks and Psychological Health: Factors Helping to Preserve Healthy Memory Performance in Normal Ageing

Abstract

Introduction: Although dementia today blights the lives of nearly 1 million people in the UK, the majority of older people will never have to live with it; but they are likely nevertheless to suffer the unwelcome loss of memory function that accompanies normal ageing. Maximising the cognitive health span of an ageing population is critically important to the efficient use of increasingly limited public health resources. In examining those factors that may preserve healthy memory performance in normal ageing, this study is not directly concerned with developing strategies to prevent or forestall dementia. However, it is likely that any memory-protective factors which help to preserve good cognitive health in normal ageing may well extend also to clinical-level memory impairment.

Methods: A large sample of cognitively normal subjects (N=145, M=55 years) underwent two separate phases of neuropsychological testing in different memory domains, including a new two-week delayed recall test of verbal episodic memory. Actigraphy was used to measure subjects’ sleep, Circadian behaviour patterns and physical activity levels over two weeks. In addition, subjects completed a range of psychological health and other questionnaires, including the Pittsburgh Sleep Questionnaire (PSQ). Factor analysis was used to reduce tested memory domains into four discrete domains: (1) short-term verbal episodic memory (2) long-term forgetting in verbal episodic memory (3) face memory and perception and (4) working memory. The same factor analysis process was used to reduce psychological health measures into three main constructs; (1) social experience (positive and negative), (2) social confidence and fearfulness, and (3) social connectedness. Comparison groups were established for age (younger, M=36 years, older, M=66 years), sleep quality (PSQ good and poor sleepers), and Circadian behaviour (owls and larks).

Results: Younger subjects outperformed older subjects in all memory domains except long-term forgetting (p=.16). Good sleepers outperformed poor sleepers in long-term forgetting (p=.0001), but not in any other memory domain. Older subjects had lower levels of Circadian dysfunctional behaviours, such as Sleep Debt and Social Jet Lag, than younger persons (p=.002 and p<.0001 respectively). However, neither Sleep Debt nor Social Jet Lag had any bearing on sleep quality (or on long term memory consolidation) and they did not differ between owls and larks (p=.18 and p=.67 respectively). Owls though tended to have larger social networks than larks (p=.007). Being more active (and less sedentary) protected working memory, and the levels of beneficial activity were different for older subjects (more light to moderate activity) than they were for younger subjects (more sustained and vigorous activity). Older subjects had better psychological health than younger subjects (p<.0001) but had no difference in social networks (p=.9). Poor sleepers had lower psychological mood than good sleepers (p=.001) but did not differ in sociability. Importantly, better psychological mood contributes to better long-term forgetting alongside good sleep.

Discussion: This study revealed a number of new findings that are important to the preservation of healthy memory performance in normal ageing. First, the double dissociation between age groups and good/poor sleep quality groups in long-term forgetting provides good evidence for the benefit of sleep to long-term memory consolidation, regardless of age. Second, while Sleep Debt and Social Jet Lag are dysfunctional behaviours, they are not necessarily embedded within ordinary chronotype differences between owls and larks, and they appear to have no relationship to sleep quality. As such, they have no overt bearing on long-term forgetting or sleep-based memory consolidation. Third, good and poor sleepers differ extensively in measures of psychological mood but do not differ in sociability, even though many of the measures of psychological mood and sociability are themselves strongly correlated. For example, poor sleepers are lonelier than good sleepers but do not have smaller social networks, although higher loneliness is strongly correlated with having smaller social networks. Also, being an owl appears to confer advantages in terms of size of friendship networks. Most importantly, good sleep and better psychological mood together predict better (lower) long-term forgetting. Finally, it is not necessary for physical activity in older age to be at similar intensity levels to that achieved by younger persons in order to secure comparable benefits to working memory. It is clear from these results that older subjects find it easier to maintain lighter levels of physical activity with increasing age, and the cognitive benefits of doing so are not to be under-estimated.