The role of sleep and shift work in dementia and cognitive aging : an epidemiological approach

Abstract

Dementia is a syndrome that afflicts older persons and is characterized by progressive deterioration in memory and other cognitive functions, behavior, and the ability to perform day-to-day activities. Older adults, and even more so those suffering from dementia, often experience disturbances in the sleep-wake cycle. This thesis sought to investigate the longterm effect of sleep and shift work (SW), which can affect sleep patterns and sleep quality, in dementia and cognitive aging using prospective and longitudinal studies of the Swedish Twin Registry (STR).

Study I prospectively explored whether sleep characteristics such as time in bed (TIB), rising time, bedtime, sleep quality, non-restorative sleep, and snoring in late life were associated with dementia incidence while accounting for baseline cognitive functioning. The study was based on a sample of 11,247 participants aged 65 and older. Short (≤6 hours) and extended (>9 hours) TIB as well as late rising time (rising 8:00 AM or later) were associated with higher dementia incidence in the following 17 years. After stratifying by baseline cognitive status, only the association between short TIB and dementia remained in those cognitively intact at baseline. Altogether, the findings suggest that extended TIB and late rising represent prodromal features whereas short TIB appeared to be a risk factor for dementia.

Study II employed quantitative genetic methods to investigate the relative importance of genetic influences for late-life sleep characteristics, dementia and Alzheimer’s disease (AD), and whether dementia-related sleep characteristics modified genetic influences on dementia and AD in a sample of 10,894 twins. Genetic influences accounted for about half of the variation in liability to dementia and AD, and for late rise time and bedtime. For the other sleep phenotypes assessed, non-shared environment contributed a larger part of the phenotypic variation. TIB and late rising were associated with dementia incidence, but these sleep traits did not moderate the genetic influences on dementia and AD.

Study III examined the association between SW and dementia incidence. The study was based on two cohorts: one cohort comprised 13,283 individuals with information on anytype SW and another cohort comprised 41,199 individuals with information on night work (NW), with follow-up time spanning 41 and 14 years, respectively. History of SW, including NW, was associated with higher dementia incidence. Further, longer duration of SW and NW appeared to be associated with greater risk of dementia.

Study IV estimated the impact of SW on change in cognitive performance before and after retirement. The study included 595 individuals at least 50 years of age at baseline who had been employed and who had undergone up to 9 assessments of cognitive performance over a period of 27 years. Latent growth curve modeling showed that SW and NW during midlife were not associated with greater rate of change in any of the cognitive domains (verbal, spatial, memory, and processing speed) later in life.

In summary, the work in this thesis has combined unique population-based data sources and modern epidemiological methods to evaluate the role of sleep and SW in dementia and cognitive aging. While SW did not appear to explain differences in rate of normative cognitive change in later life, findings indicate that SW and sleep characteristics are associated with increased risk of dementia.