Nocturnal sleep duration and cognitive impairment in a population‐based study of older adults

Objective We aimed to investigate the association between nocturnal sleep duration, changes in nocturnal sleep duration and cognitive impairment in older adults. Methods 4010 participants of a population‐based cohort study provided information on nocturnal sleep duration at baseline (1991–1995) and at follow‐up (2002/2003). 792 follow‐up participants aged 70+ by 2006 participated in telephone‐based cognitive assessments. Several cognitive tests were used including the telephone interview for cognitive status (TICS). Cognitive impairment was defined as <31 points on the TICS (13.0%) and as below this percentile on the other tests. Based on individual tests, a verbal memory score and a total score were constructed. Multivariable prevalence ratios (PRs) of cognitive impairment and 95% confidence intervals (95%CIs) were computed using Poisson regression. Analyses were restricted to those free of depression in 2002/2003 ( n  = 695). Results Sleeping ≤6 or 8 h per night ( versus 7 h) were unrelated to cognitive impairment. Sleeping ≥9 h was positively, although imprecisely, associated with impairment of verbal memory (PR = 1.7, 95%CI = 1.0, 3.0), and less pronounced with the other cognitive measures. An increase in sleep duration from 7–8 h in 1992–1995 to ≥9 h 8.5 years later ( versus sleeping 7–8 h at both time points) was associated with an increased prevalence of cognitive impairment according to the TICS (PR = 2.1, 95% = 1.0, 4.5) and the verbal memory score (PR = 2.0, 95%CI = 1.0, 3.8). Conclusions Increases in sleep duration are associated with cognitive impairment. A biological explanation for this association is currently lacking. Increases in sleep duration could be a marker of cognitive deficits. Copyright © 2009 John Wiley & Sons, Ltd.