Cognitive impairments in individuals with insomnia: clinical significance and correlates

Abstract

Introduction Although a considerable proportion of individuals with insomnia report altered cognitive performance, little is still known regarding the nature, significance and mechanisms of cognitive impairments underlying these cognitive complaints. The aims of this study were to further document the nature and clinical significance of cognitive impairment in individuals with insomnia and to investigate their correlates. Materials and methods Participants were 25 adults with primary insomnia (INS) and 16 matched controls (CTL) who underwent 3 consecutive nights of polysomnography (PSG). On the morning following night 3, participants completed a battery of questionnaires and neuropsychological tests, including the Continuous Performance Test-II (CPT-II) and the California Verbal Learning Test-II (CVLT-II). Groups were compared using MANOVAs for each of 4 cognitive domains (attention, working memory, episodic memory, executive functions). Clinically-significant deficits were defined as a performance at least one standard deviation below the normative mean. For each variable, frequencies of clinically- significant deficits in both groups were compared using Chi-square tests. Correlates of cognitive impairments were investigated using stepdown linear regressions. Potential correlates included: insomnia severity, depressive symptoms, anxiety, fatigue, sleepiness, arousal, beliefs about sleep, sleep continuity (from sleep diaries and PSG), sleep architecture and sleep microstructure variables (from power spectral analysis of the NREM sleep EEG). Results There were significant group differences for the attention (p = 0.04) and episodic memory domains (p = 0.01). Compared to CTL, INS had a poorer detectability (p = 0.01) and more perseverative errors (p = 0.03) on the CPT-II, and more intrusion errors (p = 0.03) on the CVLT-II. Clinically-significant deficits were more frequent in INS for CPT-II perseveration errors (p = 0.02) and CVLT-II intrusion errors (p = 0.03). All performance variables were significantly associated with either subjective or objective sleep continuity, and some were also independently related to sleep microstructure (i.e., relative power for 0–1 Hz and 8–12 Hz frequencies) or selected psychological variables (i.e., beliefs or arousal). Conclusion These findings suggest clinically-significant alterations in attention and episodic memory in individuals with insomnia. These deficits appear associated with sleep continuity, and may also be related to sleep EEG microstructure and psychological functioning. Acknowledgements Supported by the Canadian Institutes of Health Research.