Associations of Sleep Characteristics With Cognitive Function and Decline Among Older Adults

V. Eloesa McSorley; Yu Sun Bin; Diane S. Lauderdale

Disclosures

Am J Epidemiol. 2019;188(6):1066-1075. 

In This Article

Discussion

In a nationally representative cohort of older men and women, we found that actigraphic measures of sleep and self-reported measures of sleep have different cross-sectional and longitudinal associations with cognitive function and decline. We found that actigraphic measures of sleep disruption and quality were negatively associated with cognition measured concurrently and were associated with 5-year cognitive decline. In the cross-sectional analysis, this association did not differ between men and women, but actigraphic measures of sleep disruption were more strongly associated with 5-year cognitive decline among men compared with women. Short actigraph-measured TST was associated with higher odds of cognitive decline, but there was no evidence of a linear or U-shaped association across the full range of TST. We did not find that self-reported sleep quality (insomnia symptomology) was associated with worse cognition or cognitive decline. Perceptions of sleep duration were also unrelated to cognitive function and decline. We found some evidence that earlier diurnal phase might be associated with greater 5-year decline. Diurnal phase is a measure of circadian rhythm, and this is often shifted earlier for older adults.[25] The observed associations are consistent with sleep disruption and potentially circadian phase shift playing a causal role in cognitive decline or with there being an underlying biological process that predisposes older adults to both. However, in SOF, peak activity occurring later in the day was associated with increased incidence of MCI/dementia.[34] This difference could be due to our defining circadian phase using the midpoint of sleep, or it could be due to the age difference between women in NSHAP and in SOF.

Unlike much of the previous sleep literature, we did not find that reported sleep duration or insomnia symptoms were related to cognitive function or decline.[7–10] However, our findings are broadly consistent with results from the SOF and MrOS. In both studies, WASO was associated with concurrent cognitive impairment on both the Trail Making Test Part B and the MMSE tests.[16,18] In SOF, low sleep efficiency (TST divided by the rest interval) was also associated with lower cognitive scores on both tests of cognitive function.[16] The association between sleep disruption and cognitive decline we observed was also observed in MrOS, where WASO, low sleep efficiency, and the number of long wake episodes were associated with greater cognitive decline over an average of 3.4 years on the MMSE and the Trail Making Test Part B.[17] A comparable association was not seen in SOF for either the Trail Making Test Part B or the MMSE (K. Stone, University of California, San Francisco, personal communication, March 2018). However, in a follow-up study among a subsample of SOF participants who were given an extensive battery of clinician-reviewed cognitive tests, low sleep efficiency and longer sleep latency (time between the beginnings of the rest interval and the sleep interval) were associated with incident MCI/dementia.[15] Of note, the average age of participants in SOF (87.4 years, and 82.6 years in the substudy) and MrOS (76 years) was older than the participants in the present study (71.9 years).[15–18]

The report of different longitudinal associations in MrOS (male cohort) and SOF (female cohort) does not necessarily imply a different effect for men and women, because there are differences besides sex between the 2 cohorts, including the age distributions at the time of analyses. However, we observed significant differences between men and women in the NSHAP Sleep Study. The reasons for these differences are unknown, although a greater prevalence of obstructive sleep apnea in men versus women could play a role, given that apnea has also been associated with cognitive impairment.[35,36] Indeed, in our sample, men had a higher prevalence of apnea at follow-up than women did (21% vs. 10%). However, we did adjust for apnea in the longitudinal model. While a number of studies have reported differences in sleep characteristics between men and women,[37–39] there has been limited research on whether the effects of sleep differ by sex. The studies that do assess this difference are often among younger populations,[40,41] rely on self-reported sleep,[42,43] or deal specifically with sleep apnea as opposed to nonclinical variation in sleep.[44,45] We believe our study is the first to report differential associations of sleep on a health outcome according to sex using objective measures of sleep in a community-based sample of older adults.

A key strength of this study is the measure of cognitive function. Unlike cognitive assessments in many omnibus surveys, the MoCA-SA is a validated multidomain assessment developed to assess and track mild cognitive impairment.[19] Additionally, NSHAP's national sampling frame allows results to be generalized to the US population of community-dwelling older adults born between 1920 and 1947. Use of wrist actigraphy allows us to compare observed associations with self-perceptions of sleep duration and quality. While many studies have considered the relationship between self-reported sleep characteristics such as insomnia symptoms and sleep duration with cognition, discrepancies between self-reports of sleep and more objective measures might be pronounced in older populations.[11,46–48] Our relatively small sample size is a limitation that must be acknowledged. Findings of marginal statistical significance might reflect, in part, the sample size and insufficient statistical power to detect associations. We did not carry out an analysis of incident MCI or dementia among those with neither at baseline because that would have further reduced the sample size. While actigraphy estimates sleep from arm motion as opposed to direct measurement of brain activity, it is generally considered a valid and useful approach to objective estimation of sleep characteristics without itself affecting sleep behavior.[49] More than 3 days of actigraphy are recommended to assess sleep patterns, particularly to capture variation between workdays and weekends. However, we have found little day-of-the-week effect in this cohort of older adults.[23]

Prior evidence that sleep characteristics are indicators of future cognitive decline among older adults has relied mainly on self-reported sleep characteristics. Previous studies focused on sleep duration as an important contributor to cognitive function, with self-reported short and long durations indicative of higher risk of cognitive decline.[8,10] In contrast to previous studies, we did not find evidence that self-reported sleep duration was a significant contributor to cognitive function. These results call into question the usefulness of self-reports of sleep as measures of sleep pertinent to cognitive function. Similarly, there appears to be no evidence that insomnia is a risk factor for cognitive decline, which might be reassuring for the many older adults who report insomnia symptoms. We have shown here that measured sleep disruption is a more important aspect of sleep than duration for predicting cognitive decline. Our findings add to the evidence that sleep disruption and quality measured using wrist actigraphy are salient dimensions of sleep when considering the relationship between sleep and cognitive function at older ages. Further, we have shown that this association might be stronger among men than among women, but that unexplained finding needs replication.

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