The sample of 90 included 55 females (61.1%) and 35 males (38.9%). Their average age was 81.2 (SD 8.56), with range from 58 to 95. Seventy-seven participants were Caucasian, 12 were African-American, and one was a Pacific Islander. Twenty-five percent of the sample had less than a high school education.
Measures of sleep continuity showed a mean sleep efficiency (ratio of time in bed to time asleep) of 64.46% (SD 15.38) with a range from 23% to 89.5%. Total sleep time (TST) ranged from 62.5 minutes to 493.25 minutes, with a mean TST of 300.82 minutes (SD 95.54). Although cut-off for study inclusion was seven hours of sleep or less measured with actigraphy, seven of ninety participants slept more than seven hours during PSG nights, indicating that TST can vary from night to night. Average sleep onset latency (minutes from lying down in bed to beginning of sleep) was 29.48 minutes (SD 34.48) with a range from 0.25 – to 194.25. Usual sleep onset latency is 15 – 25 minutes. Number of awakenings ranged from one to 173 with an average of 36.89 (SD 27.31). Minutes of wake after sleep onset averaged 130.04 (SD 64.31) and ranged from 15.75 – 333.00. The average time in bed for participants was 460.61 (SD 92.47) and ranged from 193.5 to 654.75.
The mean MMSE score was 20.54 (SD 7.33), with scores ranging from 4 – 28 indicating mild to severe cognitive impairment. Of the 90 participants, 9 were diagnosed with Alzheimer’s disease and had a mean MMSE score of 15.11 (SD 6.53); 6 had multi-infarct dementia with a mean MMSE score of 18.67 (SD 8.04); 19 had other types of dementia (alcoholic, Pick’s, Lewy body) and a mean MMSE score of 23.53 (SD 4.8); 26 had unspecified dementia (mean MMSE 16.31 [SD 7.27]), and 26 were diagnosed with mild cognitive impairment (mean MMSE 26.23 [SD 2.54]).
The mean level of assistance score was 20.68 (SD 4.41): scores ranged from 7 to 24 indicating that a majority of participants were unable to follow multi-step instructions. All participants were ambulatory and took an average of 33.47 (SD 48.16) seconds to walk 6 meters. Gait speed ranged from a surprisingly brisk 3.49 seconds to 282 seconds. presents the means and standard deviations for sleep, cognitive status, and functional status variables for the 90 participants.
Descriptive Statistics for Sleep, Cognitive status and Functional Status
To describe the relationships among disordered sleep, cognitive status, and functional status, we conducted Pearson product-moment correlations. summarizes these correlations. Because of skewed data we also conducted Spearman’s rho correlations; however no change in statistical significance was observed. The non-parametric procedure was explored because of concern regarding violation of assumptions underlying r.
Correlations among sleep, cognitive status, and functional status variables.
Decreased total sleep time (TST) (r = −0.21, p = 0.04) and fewer awakenings (r = −0.25, p =0.02) were associated with better cognitive status. Decreased TST was also associated with better functional status measured as level of assistance required (r = −0.30, p= 0.01) and functional status measured as gait speed (r = 0.32, p=0.01). Better cognitive status was strongly associated with functional status measured as level of assistance required (r = 0.68, p=0. 01) and with better functional status measured as gait speed. (r = − 0.40, p= 0.01). After controlling for the effect of cognitive status, only the association between better gait speed and decreased TST remained significant (r = 0.24, p=0.03) ( summarizes partial correlations controlling for cognitive status).
Partial correlations controlling for cognitive status
To provide additional description of the relationships among sleep and cognition we created three MMSE categories (0 – 10 [severe cognitive impairment], 11 – 20 [moderate cognitive impairment], and 21 – 29 [mild cognitive impairment]). We calculated the mean sleep parameters for each group () and compared means using a one way analysis of variance. We found no statistically significant differences, although the differences in mean TST among groups showed a trend toward significance (F (2, 87) = 2.87, p < 0.06).
Mean sleep parameters by MMSE category
Finally, awakenings were categorized by type (respiratory [55%], spontaneous [39%], bathroom [5%], and periodic limb movements [1%]). Further analysis determined that fewer respiratory awakenings and higher SaO2 nadir were significantly associated with better cognition (r = −0.24, p < 0.03; r = 0.23, p< 0.04). In this sample, 60.8% had an apnea-hypopnea index (apneas and O2 desaturations ≥4% per hour) greater than 5, indicating a diagnosis of obstructive sleep apnea.