In this study, in the SDSC questionnaires, DA and DOES subscales were significantly related with developmental aspects (cognition, fine motor, language, and social awareness) in children with DD. However, tests for the gross-motor function and functional abilities were not associated with SDSC questionnaires.
Several studies have evaluated the relationships between sleep problems and developmental aspects in typically developing infants and toddlers.[7, 8]
Studies regarding sleep problems in children with developmental problems have also been reported.[3, 15] Additionally, some studies have assessed the relationship between sleep problems and their associated developmental aspects.[16–18] However, these studies are generally limited in specific disease groups such as autism spectrum disorder and cerebral palsy.
To the best of our knowledge, researches regarding these relationships in toddlers and preschoolers with DD including various disease entities have not been published. For preschool-aged children, their life in school is the beginning of socialization and is important for development. Therefore, we aimed to evaluate the relationships between the two targeting children during this period. Moreover, we conducted the study using assessment tools commonly used in clinical practice for a long period not designed for research so that parents’ understanding of the tool could be warranted, which is crucial in identifying the developmental status of children.
The study demonstrated a significant correlation between DA in the SDSC questionnaire and language expression in BSID-Ⅲ, with DA appearing to have a significantly negative impact on language expression in children with DD.
Generally, sleep is considered to have an important role in language learning and communication skills.[19] It may be required for long-term retention after learning and improvement in the acquisition and integration of vocabulary in school-aged children.[20]
The day-and-night sleep duration ratio were also correlated with language outcomes, suggesting that early mature sleep consolidation could be helpful to prevent language delays in developing children.[21]
In addition, sleep cycles or stages are thought to influence language learning, such as vocabulary acquisition.[22] For example, in a previous research regarding the relationship between sleep disturbance and language deficits in toddlers with DS, the authors suggested that a specific consolidation process during slow-wave sleep (SWS) might contribute to these correlations based on their findings that difficulties with expressive language such as vocabulary and syntax were observed in children with DS with poor sleep quality.[20]
According to the International Classification of Sleep Disorders, third edition (ICSD-3), sleep walking and night terrors are subtypes of NREM parasomnia and correspond to disorder of arousals.
The DA domain of the SDSC questionnaire contains not only NREM sleep parasomnias but also nightmares that mainly occur during the REM sleep. A study demonstrated that individuals experiencing nightmares exhibit poor sleep quality and altered sleep architectures including relatively shortened SWS and NREM duration.[23]
Although the exact mechanism is not clearly understood, it might be related to the language learning process and recall, thereby affecting the language expression. Further research will be needed to establish a more precise understanding of this relationship.
Meanwhile, DD itself seems to precipitate sleep disorders. More children with DD experience sleep disturbances than normally developing children.[2]
The rates and nature of sleep difficulties are different among the groups of children with DDs. However, regardless of the groups, their sleeping difficulties might be related to their behavioral difficulties.[24] Considering the effects of sleep on language learning mentioned above, language skills may be further deteriorated in children with DD and sleep problems as a comorbidity.
Among the SDSD questionnaire domains, DOES was significantly associated with cognition and socio-emotional scales of BSID-Ⅲ. This result suggest that excessive somnolence might be an important factor that negatively affects the development of children.
Excessive somnolence, a type of hypersomnia, is a condition characterized by falling asleep during the day repeatedly and excessively. It can be caused by sleep deprivation, obstructive sleep apnea, sedating medications, and medical, psychiatric, or neurologic disorders.[25]
Children with hypersomnia have lower quality of life and education problems than healthy controls as well as more mood and behavioral problems, which lead to negative psychosocial consequences.[26] Our study demonstrated a notable correlation between DOES and socio-emotional and cognition domains, respectively, aligning with findings of previous studies mentioned above.
Only a few previous studies have reported the relationship between sleep problems in preschool-aged children and cognition and socio-emotional aspects.
A recent systemic review demonstrated that although few studies are included, the association between sleep problems and cognition and behavior was identified, showing that these associations could be observed as early as preschool-aged child. The correlation was mostly found in sleep quantity such as night time sleep duration and quality.[27]
Meanwhile, studies regarding the relationship between excessive daytime sleepiness (EDS) and social behavior were scarce, especially in preschool-aged children.
A recent cross-sectional study in Italy revealed that EDS was negatively correlated with prosocial behaviors and could affect teacher–student relationships in kindergarten children.[28] Furthermore, a national population study conducted with Australian preschool-aged children indicated a connection between sleep problems and health-related quality of life (HRQoL), particularly in the psychosocial aspect. Notably, morning tiredness was associated with the poorest HRQoL.[29]
Our study showed similar relationships in preschool-aged children with DD. However, conflicting results of the study also exist, demonstrating that daytime sleepiness was not significantly associated with daytime behavior.[30] As objectively measuring DES is difficult especially in preschool-aged children and mostly rely on reports from their parents, this could be the reason for the somewhat different study results.
This study has potential limitations. First, as it is a cross-sectional study, the temporal relationship between sleep disorders and the children’s development cannot be determined, and thus, the long-term effects of variables cannot be assessed. Second, factors such as medication were not controlled so that the possibility that such factors may have affected sleep and development respectively cannot be ruled out. Third, the study was limited by a relatively small sample size and lack of the control group that may influence the research finding and may be hard to generalize. Future study with larger sample size will be needed.
Fourth, in preschool-aged children, objective sleep measurement is difficult; therefore, most studies mostly rely on reports from or questionnaire answers of their caregivers or teachers. Thus, future researches using objective methods to measure the quality and quantity of sleep in children will be needed.