Cognitive functioning in the community elderly : the role of sleep and caffeine

Abstract

Deteriorations in cognitive functioning and sleep are the inevitable parts of the ageing process, and they are two very common complaints among the elderly population. Given their high relevance and great impact on daily functioning, many studies have attempted to address the associations of sleep problems and cognitive functioning with ageing, yet the direction of associations remained unclear. Several recent studies suggested that caffeine, a common psychostimulant present in coffee and tea, might have a beneficial effect on agerelated decline in cognitive functioning. Nonetheless, the dose-dependent effect of caffeine intake on specific domains of cognitive functioning, and the potential cost of compromised nocturnal sleep at high dose of caffeine remained to be investigated. In view of the lack of study on identifying the correlation and interaction between sleep, cognitive functioning, caffeine consumption habit, and age, this thesis reported two studies that clarified these relationships in the elderly population. Furthermore, the studies explored the possibility to utilize sleep and caffeine as a regimen to improve daytime cognitive functioning in the older population.

Study 1 was a retrospective study that aimed to examine the relationship between sleep, habitual caffeine consumption, cognitive functioning, and mood in the two different age groups, the young adult and the elderly. Eighty-nine healthy elderly and forty-three healthy young adults participated in this study and completed a battery of neuropsychological assessment and a set of questionnaires. Findings revealed changes in multiple domains of sleep and cognitive functioning upon ageing. The age-related differences in sleep and cognitive functioning were correlated. Moreover, result suggested that regular caffeine consumption has a dose-dependent beneficial effect on cognitive functioning, but the effect was only observed in female.

Study 2 was built on the relationship between sleep, cognitive functioning, and caffeine as found in Study 1. It aimed to investigate the effect of caffeine and daytime nap on the cognitive functioning in the healthy elderly adopting a repeated measure, double-blind, placebo-controlled design. Twenty-four healthy elderly were recruited for this study, and each of them were required to attend four experimental sessions with a one-week intersession interval. In each session, participants were required to take a rest or take a nap with or without a certain dosage of caffeine in the afternoon. Comparisons on their cognitive performance before and after the rest/ nap revealed an effect of nap and caffeine on improving subjective feeling of sleepiness and fatigue. Behavioural measurements revealed no effect on daytime nap on cognitive functioning, yet specific sleep stage and certain sleep oscillations were associated with post-nap changes in cognitive functioning. In summary, the present studies demonstrated the associations of sleep and caffeine consumption with cognitive functioning in the elderly. Habitual caffeine consumption was associated with a female-specific beneficial effect on cognitive functioning. Furthermore, daytime nap combined with the use of acute dose of caffeine might not enhance cognitive functioning, but could improve mood and well-being in the elderly. Findings from present studies suggested that further research could explore ways to maximise the benefit of napping in the elderly.