Sleep problems in children with dyslexia: understanding the role of sleep in neurocognitive development through the lens of developmental disorders.

It has long been recognised that sleep is more than a passive recovery state or mere inconvenience. Sleep plays an active role in maintaining a healthy body, brain and mind, and numerous studies suggest that sleep is integral to learning and memory. The importance of sleep for cognition is clear in studies of those experiencing sleep deprivation, who show consistent deficits across cognitive domains in relation to nonsleep-deprived controls, particularly in tasks that tax attention or executive functions (1). Poor sleep has been associated with poor grades, and academic performance suffers when sleep is sacrificed for extra study (2). Thus, it is perhaps unsurprising that children with developmental disorders of learning and cognition often suffer from sleep disturbances. These have been well documented in children with autism and attention-deficit/hyperactivity disorder (ADHD), where sleep problems can be particularly severe. However, a growing body of evidence suggests that sleep can be atypical across a spectrum of learning disorders. Understanding the ways in which sleep is affected in different developmental disorders can not only support the design and implementation of effective assessment and remediation programs, but can also inform theories of how sleep supports cognition in typical development. The study by Carotenuto et al. (3) in this issue makes a valuable contribution to this literature by looking at sleep disturbances in children with developmental dyslexia. Dyslexia is the most common specific learning disorder, affecting around one in 10 children in our classrooms. It is characterised by difficulties with reading and spelling and is primarily caused by a deficit in phonological processing. However, dyslexia often co-occurs with other developmental disorders, such as ADHD and specific language impairment, and there can be striking heterogeneity between children. This has led to the suggestion that dyslexia can result from complex combinations of multiple risk factors and impairments (4). Consequently, research attention is turning towards the wider constellation of subclinical difficulties often experienced by children with dyslexia, including potential sleep problems. Two preliminary studies have found differences in the sleep architecture of children with dyslexia in comparison with typical peers, using overnight sleep EEG recordings (polysomnography) (5,6). Notably, children with dyslexia showed unusually long periods of slow wave sleep and an increased number of sleep spindles. Slow wave sleep and spindles are related to language learning, most notably through promoting the consolidation of new vocabulary (7). Children with dyslexia have pronounced deficits in learning new oral vocabulary, providing a plausible theoretical link between sleep disturbances and language difficulties. If sleep problems do in fact exacerbate the learning difficulties associated with dyslexia, as well as impacting on daily cognitive function, this could have important implications for intervention and support programs. However, an important first step is to establish the nature and extent of sleep disturbances in dyslexia. Previous studies (5,6) have used small samples (N = <30) and examined a large array of sleep parameters on a small number of unusual nights (where children were wearing sleep recording equipment), as opposed to looking at global patterns over time. As such, how representative these findings are is questionable, and consequently these studies should be viewed as hypothesis-generating rather than hypothesis-testing. Carotenuto et al. (3) address some of these concerns, administering questionnaire measures of sleep habits to the parents of 147 children with dyslexia and 766 children without dyslexia, aged 8–12 years. A sample of this size allows for a robust analysis of sleep characteristics. Therefore, their findings that children with dyslexia showed higher rates of several markers of sleep disorders lend significant weight to suggestions that dyslexia might be associated with an increased risk for sleep problems. Importantly, the sleep questionnaire used by Carotenuto et al. (3) allows for a breakdown of sleep disturbances. It is interesting to note that they found the greatest difficulties in initiating and maintaining sleep, sleep breathing disorders and disorders of arousal. This closely mirrors the types of sleep problem documented in children with ADHD (8). While Carotenuto et al. (3) took care to exclude children with comorbid diagnoses, many children with dyslexia show subtle features of attention disorders that do not reach clinical thresholds. Future studies that can establish whether sleep disturbances are associated with subclinical attention problems or dyslexia per se will be particularly informative for understanding which cognitive skills most critically relate to sleep. This is also vital information for