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This study provides the first meta-analysis of the purported differences in sleep time and sleep quality between people with and without intellectual disabilities. Twenty-one papers were identified that compared sleep time and/or sleep quality in people with and without intellectual disabilities. The meta-analysis of sleep time revealed that people with an intellectual disability slept for 18 min less, on average, than people without an intellectual disability. This significant difference was limited to those studies that tested groups of people with an identified genetic syndrome or developmental disorder. The analysis of sleep quality also concluded that people with intellectual disabilities experienced poorer sleep: In 93% of comparisons between groups, sleep was found to be of poorer quality in the group of people with intellectual disabilities. There were no differences found between studies that measured sleep objectively and those that used diary or questionnaire measures. Notably, most samples were drawn from populations of people with specified genetic syndromes or developmental disorders, rather than intellectual disability of heterogeneous origin. Similarly, most studies investigated sleep in children, although there was no evidence that the differences between the groups reduced during adulthood. Most studies used highly-regarded objective measures of sleep, such as polysomnography or actigraphy, although methodological flaws were evident in the identification of samples and the measurement of intellectual disability.
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Background Intellectual disabilities are defined by co-occurring deficits in intellectual ability (often defined as an IQ < 70>
Corresponding author. Cerebra Centre for Neurodevelopmental Disorders, School of Psychology, University of Birmingham, Edgbaston, B15 2TT, UK. E-mail address: adrsurtees@gmail.com (A.D.R. Surtees). Contents lists available at ScienceDirect Sleep Medicine Reviews journal homepage: www.elsevier.com/locate/smrv https://doi.org/10.1016/j.smrv.2017.11.003 1087-0792/© 2017 Elsevier Ltd. All rights reserved. Sleep Medicine Reviews 40 (2018) 135e150 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. Challenges in researching sleep in people with intellectual disabilities Understanding and defining sleep 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 sleepewake 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). Individual differences and potential confounds 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. T
his 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], PradereWilli 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.
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