Do sleep complaints contribute to age-related cognitive decline?

The cognitive changes that occur with ageing are usually referred to as 'age-related cognitive decline'. The most pronounced changes may be found in the executive functions that require integrity of the prefrontal cortical circuitry. With age, sleep also changes profoundly, with more sleep fragmentation, earlier awakenings and less slow wave sleep as its main features. Interestingly, experimental sleep deprivation studies in healthy young adults showed a particularly consistent effect on executive functions, suggesting that sleep problems might contribute to the cognitive changes accompanying older age. We here investigate this possibility by reviewing reports on age-related and insomnia-related changes in cognition and brain function and structure, as found in studies investigating subjective complaints, objective functioning in everyday life, neuropsychological assessment, psychometry, structural and functional magnetic resonance imaging, electroencephalography, positron emission tomography and transcranial magnetic stimulation. The chapter focuses on the 'normal' age-related sleep changes that are experienced as insomnia - that is, fragmentation of sleep, more superficial sleep, more wake after sleep onset and earlier awakenings - rather than on specific sleep disturbances as sleep-disordered breathing, restless legs or periodic limb movements during sleep, for all of which the risk increases with age. It turned out that relatively few studies directly addressed the question whether elderly with different degrees of sleep complaints are differentially affected by 'age-related cognitive decline'. Still, several similarities between age-related and insomnia-related cognitive and brain changes are apparent, notably with respect to performance requiring integrity of the prefrontal cortical system. We suggest that at least part of what we regard as age-related changes may, in fact, be due to poor sleep, which is in some cases a treatable condition. Further research directly comparing aged good sleepers versus aged insomniacs will need to elucidate how sleep disturbances are involved in the cognitive, structural and functional changes observed with increasing age. The findings suggest that discrimination of subtypes of poor sleep at high age will aid in understanding the mechanisms by which it affects cognition and brain function.