Clinical reviewSleep duration and sleep quality in people with and without intellectual disability: A meta-analysis
Introduction
Intellectual disabilities are defined by co-occurring deficits in intellectual ability (often defined as an IQ < 70) and deficits in day-to-day functioning [1], [2]. An estimated 2–3% of people in the United States are thought to have an intellectual disability [3]. These people are by no means a homogenous group, with causes of intellectual disability including genetic syndromes, pre-natal complications, perinatal insult and later brain injury [4]. As well as deficits in intellectual ability and functioning, people with intellectual disability are at risk of a range of comorbid psychological and social difficulties [4], and of physical health and other developmental problems [5].
Reports of experienced clinicians and a growing body of empirical evidence support the idea that children with intellectual disabilities more regularly experience poorer sleep than typically developing children [6], [7], ∗[8], [9], ∗[10], ∗[11], ∗[12]. The smaller body of evidence on sleep in adults with intellectual disabilities presents similar findings ∗[13], ∗[14], ∗[15]. In spite of the growing number of studies conducted with these populations, reviews have suggested that there are significant limitations to knowledge of how sleep differs in people with and without intellectual disabilities [10]. Methodological differences between studies have made the prevalence of sleep problems hard to quantify, with estimates ranging from 13% to 86% [8]. Such wide variability in estimates makes it difficult to quantify the scale of the problem with confidence and direct resources to those in most need. Among the most common reported problems with sleep in this population are shorter durations of sleep time and lower scores on various indices of quality of sleep [16], [17]. One way of understanding sleep problems in this population, therefore, is to quantify the differences in sleep time and sleep quality between this group and people without intellectual disabilities.
The current systematic review includes the first meta-analysis of studies comparing sleep in people with and without intellectual disabilities. The search terms used identified measures of sleep time, estimates of sleep quality and frequency of sleep problems.
One reason for the variability in findings across studies of sleep in people with intellectual disabilities has been the limitations to our understanding of sleep more broadly. The science of sleep has been developing rapidly over the past 30 y [18], and large variability in sleep exists in people with seemingly healthy sleep patterns [19]. Unhealthy sleep has been categorised into a range of identified sleep disorders. The international classification of sleep disorders- 3rd edition lists six broad categories of these: insomnia, sleep-related breathing disorders, central disorders of hypersomnolence, circadian rhythm sleep–wake disorders, parasomnias and sleep-related movement disorders [20]. There is some evidence that a broad range of sleep disorders are more prevalent in people with intellectual disabilities [8], but also evidence that the variability in quality of sleep can extend beyond the presence of a specified disorder [8]. Whether meeting diagnostic criteria for a sleep disorder or not, poor sleep can have an impact on people and their families and poor sleep is often inferred based on either shortened sleep time or reduced sleep quality [8]. In this review, sleep time and sleep quality are treated as distinct dependent variables. Advantages of this include allowing for broad conclusions and reducing the chance of type-1 error (through making fewer comparisons). One disadvantage is that sleep quality has been considered in numerous ways and therefore is not necessarily a homogenous construct.
Sleep time as a lay construct is defined in different ways-varying from the total amount of sleep across a day to the longest period of sleep within a day. In studies that measure sleep objectively, the construct of total sleep time (TST) is typically used. This includes the total time spent asleep over a defined night sleep period. Shorter sleep time has been associated with poor functional outcomes for people with intellectual disability, including more sleepiness the following day and increased challenging behaviour [21], [22]. For this reason, we have considered group differences evidencing shorter sleep time to reflect poorer sleep within that group, though we acknowledge that ideal sleep time may differ from one individual to the next. For the meta-analysis, measures of sleep time will include TST from objective measures of sleep, reported hours of sleep in 24 h (where TST not available) and reported hours of sleep from diaries (as appropriate).
Sleep quality is perhaps even harder to define and it is broadly accepted that sleep quality is not equivalent to sleep time, although the two are related ∗[8], [23]. For instance, fractured sleep time and regularly waking may compromise sleep quality beyond reducing the overall duration of sleep [8]. Our strategy for the current paper has been to include a measure of sleep quality where possible, acknowledging that this may mean combining studies that did not measure the same aspect of sleep quality (see discussion in Method).
Most research on sleep in people with intellectual disability has been undertaken with children [24]. Most parents of this group report that they believe poor sleep in their children is an inevitable result of intellectual disability or brain damage [9]. However, people with intellectual disabilities are by no means a homogenous group, so consequently problems with sleep are likely of varied aetiology [25]. Factors such as severity of intellectual disability ∗[14], [24] and poor social and communication skills [9] have been linked to poor sleep. This suggests that people with intellectual disabilities may be more or less likely to experience poor sleep depending on the degree of their impairment. Similarly, as well as these functional impairments, people with intellectual disabilities are more likely to have physical health conditions, such as epilepsy, posited to have independent mechanisms that would predict poor sleep [26], [27]. In addition to physical health conditions, people with intellectual disabilities are thought to be more likely to experience chronic pain, that in many cases is unrecognised and untreated [28], which greatly increases the likelihood of problems with sleep [29].
People with intellectual disabilities are also more likely to have comorbid genetic and developmental disorders, many of which have been associated with poorer sleep. People with Smith Magenis syndrome often evidence inverted melatonin cycles [30], predictive of difficulties with sleep at night and difficulties maintaining wake and attention in the day time. There is evidence of a significantly increased prevalence of problems with sleep in people with Down syndrome [31], perhaps associated with the physical differences linked to the condition predisposing sleep disordered breathing and sleep apnoea [32]. Additionally, poorer sleep is reported in children with Angelman syndrome [33], Williams syndrome [34], fragile-X syndrome [35], Prader–Willi Syndrome [36], Rett syndrome [37], Sanfilippo syndrome [38] and Jacobsen syndrome [39]. Sleep in people with autism spectrum disorders (ASD) has received more attention than other developmental disorders [10]. Interestingly, increased autistic symptomology predicts an increased likelihood of problems with sleep [40], [41]. There is also some evidence that intellectual disability is further predictive of poorer sleep in people with ASD [11]. Given these reports, in the current study, sleep in people with intellectual disability of heterogeneous origin is examined separately from that in people with genetic syndromes/developmental disorders as well as together.
Evidence for poor quality sleep in people with intellectual disabilities is often drawn from parent reports [8], or reports of staff in care homes [14], rather than objective measurement. This clearly reduces methodological load on experimenters, individuals, their families and staff, often allowing for larger sample sizes. However, there is conflicting, yet significant, evidence showing parents of typically [42] and atypically [43] developing children overestimate their child's sleep difficulties. Similarly, there is evidence of poor concordance between parent report and actigraphy in other developmental disorders, such as attention deficit hyperactivity disorder (ADHD) [44]. Evidence is less clear for adults with intellectual disabilities. Additionally, questionnaire measures, the most common tool for such studies, are rarely validated for populations of people with intellectual disabilities [10]. Similarly, most samples have been, at least to some degree, self-selecting (although systematic cohort samples have been collected [9], ∗[13]). In the current review, supplementary analyses investigate whether any group differences are reflected when only studies using objective measures are analysed. Furthermore, a set of independent quality criteria against which to weight studies based on the strength of methodology is implemented.
Over the past 25 y, there have been a significant number of studies reporting comparisons of sleep time and sleep quality in populations of people with and without intellectual disabilities. However, these studies typically have comparatively few participants and often make methodological compromises, such as offsetting the benefits of sample size against depth of data gathering. Additionally, groups of people with intellectual disabilities are not homogenous entities. This meta-analysis, examining the evidence for whether people with intellectual disabilities do have a shorter duration and poorer quality of sleep than people without intellectual disabilities, is timely. We provide further, exploratory analysis on the relative effect on any identified difference of independent variable factors (such as inclusion of people with ASD/genetic disorders), dependent variable factors (such as hours slept vs. sleep quality) and experimental design factors (such as parent report vs. objective measurement). The introduction of independent quality criteria allows for the impact of studies to be weighted based on their reliability and validity.
Section snippets
Search strategy
A systematic literature search was conducted using the databases Medline, Embase and Psychinfo in June 2015. All search terms were adapted from van de Wouw and colleagues' recent systematic review of sleep in adults with intellectual disability [14]. This recent review was aimed only at adults and specifically at sleep problems, but also included more generic terms for sleep. For full search terms see Supplementary materials (S1). Terms for intellectual disability included: intellectual
Participant characteristics
In spite of the search returning over 1500 papers, only 21 were included in the final analyses. In addition, five studies only reported comparisons of the frequency of sleep problems between groups of people with and without intellectual disabilities; these are considered at the end of the results section (and included in Table S2). The 21 papers in the analysis included 33 groups of people with intellectual disabilities (see Table S2 for a full description of the papers, including participant
Summary
Research on sleep in people with intellectual disabilities and/or developmental disorders has blossomed over the last 15 y [12]. The viability of this meta-analysis highlights that fact, including data from 26 different studies in which authors compared the duration and/or quality of sleep in people with intellectual disabilities to those without intellectual disabilities. Findings of the meta-analysis suggested that there was evidence to support the hypothesis that people with intellectual
Conclusions
More than 30 y of research has suggested that people with intellectual disabilities experience poorer quality and shorter duration sleep than their typically developing peers. This is the first meta-analysis of the literature to examine this research question. This analysis suggests that both conclusions are supportable, to some degree. Significant limitations exist, most notably the proportion of research based on child participants and the lack of studies based on people with intellectual
Conflicts of interest
The authors declare no conflict of interest in relation to this work.
Acknowledgements
This work was conducted at the Cerebra Centre for Neurodevelopmental Disorders, University of Birmingham and was supported by funding from Cerebra. Many thanks to Anna Joyce and two anonymous reviewers for comments on previous drafts of this article.
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