Clinical ReviewInsomnia and daytime cognitive performance: A meta-analysis
Introduction
Insomnia is a widespread health problem, with recent reports estimating that 25–30% of adults in the general population experience occasional sleep problems, while 10% suffer from sleep disturbance severe enough to meet diagnostic criteria for insomnia.1, 2, 3 As a chronic condition, insomnia entails significant personal and social costs: individuals with insomnia consistently report decreased quality of life4 and are at greater risk for depression.5 Insomnia is also associated with higher rates of absenteeism and loss of productivity,6, 7 and recent health economic analyses suggest that more than 90% of insomnia-related costs result from these occupational consequences.8 The burden of insomnia thus lies predominantly in the impact it produces on daytime functioning rather than in nighttime sleep disturbances per se.
The most commonly reported daytime symptoms in individuals with insomnia are fatigue and mood disturbances, experienced as difficulty handling minor irritations, reduced interest and decreased satisfaction in leisure activities and relationships.*9, 10, 11 Complaints related to altered cognitive functioning are also frequent and involve memory and concentration problems, difficulty making decisions and frequent work-related mistakes.*9, 10, 11 However, these complaints have not been unequivocally corroborated by objective performance-based measures. Over 30 studies have been published on the matter but only a small proportion has found differences between individuals with and without insomnia. For example, a recent review suggests that performance of individuals with insomnia and normal sleepers is comparable for different tasks assessing psychomotor function (e.g., finger tapping, Purdue Pegboard), some aspects of attentional function (e.g., Digit Symbol Substitution Test, vigilance tasks, Trail Making Test) and episodic memory (e.g., Hopkins Verbal Learning Test, Verbal Paired Associates, Auditory Verbal Learning Test).12 On the other hand, tests measuring working memory (e.g., Digit Span, Letter-Number Sequencing) and executive function (e.g., Wisconsin Card Sorting Test, verbal fluency, maze tasks) have yielded contradictory findings. The lack of consistent evidence has led some to question the existence of daytime cognitive impairments in insomnia 13 and to attribute daytime complaints to other mechanisms such as excessive attention toward expected consequences of poor sleep.14
While these equivocal findings could suggest that cognitive functioning is well preserved in individuals with insomnia, they could also result from a number of methodological issues related to lack of statistical power, heterogeneity of subjects, and use of insensitive measures to detect mild defitics. Indeed, many of the studies in this domain were conducted with fairly small samples (<20 participants per group), which may have limited statistical power to detect subtle differences between normal sleepers and individuals with insomnia. In addition, participants in some of these studies were not always adequately characterized with regard to insomnia diagnosis and severity of sleep disturbances (e.g., unclear selection criteria, lack of valid diagnostic measures), thus potentially introducing heterogeneity in the samples and further decreasing statistical power. Also, the use of measures not specifically designed for this population may preclude adequate sensitivity to detect cognitive deficits. Given these limitations, it appears premature to conclude that the lack of consistent significant group differences in available studies (i.e., negative findings) is evidence of preserved cognitive functioning in individuals with insomnia. This meta-analysis was intended to circumvent the problem of low statistical power by pooling individual studies and conducting a quantitative review of the evidence. The aim was to estimate the magnitude of differences in cognitive performance between individuals with insomnia and normal sleepers.
Section snippets
Identification of eligible studies
The MedLine, PsycInfo and Dissertation Abstract International databases were searched using “insomnia” or “sleep initiation and maintenance disorders” as keywords or major descriptors. The results were then crossed with the following keywords: “cognitive”, “assessment”, “psychological tests”, “psychomotor performance”, “performance”, “neuropsychology”, “memory”, “attention”, “cognitive disorders”, “neurobehavioral manifestations” or “mental processes”. The search was conducted in January 2009.
Flow of included studies
The flow chart of study selection is presented in Fig. 1. Of the 866 studies initially retrieved using the search criteria, 24 were included in the meta-analysis. A total of 627 papers were excluded from the title or abstract (i.e., were not empirical papers, did not include a sample of individuals with primary insomnia, did not report a cross-sectional comparison to individuals without sleep problems, etc.). Among the remaining studies, the main reasons for exclusion were: being written in a
Discussion
The aim of this meta-analysis was to summarize evidence regarding the magnitude of differences in cognitive performance between individuals with primary insomnia and individuals without sleep disturbances. Results suggest that insomnia is associated with mild to moderate impairments on specific cognitive functions. Individuals with insomnia performed more poorly on tasks assessing working memory (retention and manipulation), episodic memory and problem solving. They also tended to exhibit a
Acknowledgements
Research supported by a doctoral fellowship awarded to Émilie Fortier-Brochu, M.Ps., by the Canadian Institutes of Health Research.
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