ReviewConnections between sleep and cognition in older adults
Introduction
As people age, changes in sleep patterns often occur, including a decrease in total sleep duration and efficiency, an increase in sleep fragmentation, greater difficulty falling asleep, and less time in rapid eye movement (REM) sleep and slow wave sleep.1, 2, 3 Although some of these changes might be a function of normal ageing, others might result from underlying medical conditions4, 5, 6, 7 or a neurodegenerative process.2 The sleep–wake cycle is regulated by complex interactions among brain regions and neurotransmitter systems8 (figure 1), many of which are implicated in memory and cognitive function.9, 10, 11 Perhaps as a result of this shared circuitry, sleep problems are common in people with Alzheimer's disease and other dementias9, 12, 13, 14, 15, 16 (panel). In adults with Alzheimer's disease, circadian rhythm dysfunction and so-called sundowning (increased confusion and restlessness at the end of the day and into the night) are frequently reported and are thought to result from degeneration of the suprachiasmatic nucleus (known as the master clock regulating circadian rhythms) and the cholinergic neurons of the nucleus basalis of Meynert.9, 15 Similarly, REM behavioural sleep disorders often accompany dementia with Lewy bodies and might indicate degeneration of brainstem structures, although the specific anatomical regions have not been determined.15, 16, 17, 18, 19, 20 Although an increased prevalence of sleep disturbances in people with dementia has been well documented and is often thought to result from neurodegeneration, focus has turned towards the possibility that sleep disturbances can also increase the risk of cognitive decline and dementia.
The aim of this Review is to discuss research investigating how sleep disturbances in older adults (≥55 years) might predict or contribute to cognitive decline and dementia. We present evidence from observational studies of the risk of cognitive decline and dementia associated with sleep disturbances in older adults, including studies of insomnia, sleep quality, sleep duration, excessive daytime sleepiness, sleep-disordered breathing, and circadian rhythm disturbances. We then discuss potential biological mechanisms underlying the association followed by results from treatment trials assessing potential cognitive benefits of improved sleep.
Section snippets
Insomnia
Insomnia is characterised by chronic problems with falling asleep, staying asleep, or poor sleep quality that occurs for 1 month or longer, causes substantial functional impairment, does not occur solely in the context of another sleep or mental disorder, and is not associated with substance use or a medical comorbidity.21, 22 An estimated 25% of older adults have either insomnia or insomnia symptoms.1, 23 In cross-sectional studies of mainly middle-aged (mean age 47 years [SD 15]) adults,24
Mechanisms underlying the link between sleep and cognition
Several mechanisms could underlie the link between poor sleep quality and cognitive impairment (figure 2). Sleep is required for the consolidation of memory and normal brain functioning,10, 97, 98 and sleep disturbance could interfere with the function of neuronal pathways, especially those of GABA and cAMP, which in turn might impair synaptic plasticity.99 Evidence from both epidemiological and experimental studies suggests a potential bidirectional association between amyloid-β accumulation
Treatment trials
Many treatments effectively improve sleep in older adults, but trials assessing cognitive benefits of the treatment of sleep disturbances in older adults are scarce. Findings from a small study in older adults (65–85 years) with insomnia showed improvements in both sleep quality (sleep onset latency and sleep efficiency) and cognitive performance after 8 weeks of a computerised cognitive training programme.124 Exercise has shown potential for improvement of sleep and wellbeing in older adults
Conclusions and future directions
Sleep disturbances are common in people with cognitive impairment and dementia. Observational studies suggest that sleep disturbances (particularly sleep duration, sleep fragmentation, and sleep-disordered breathing) might increase the risk of cognitive impairment. Some, albeit less consistent, evidence exists for associations of cognitive impairment with insomnia and circadian rhythm dysfunction. These findings suggest that the sleep–wake cycle plays a crucial part in brain ageing, pointing to
Search strategy and selection criteria
We identified references for this Review by searches of PubMed between Jan 1, 2002, and March 13, 2014, and references from relevant articles. We used the search terms “sleep”, “insomnia”, “sleep quality”, “sleep duration”, “sleep disordered breathing”, “sleep apnoea”, “circadian rhythm” and “dementia”, “Alzheimer's disease”, “cognitive decline”, “cognitive function”, or “cognition”. Articles were restricted to those available in English. We made the final selection by using the most relevant
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