Introduction to the special issue: sleep in children with neurodevelopmental and psychiatric disorders.

For those clinicians and researchers who have chosen and embraced the challenges and rewards of working on a daily basis with children and families, the concept of ‘‘developmental context’’ is hardly a revelation. As with virtually any issue that pediatric practitioners confront in clinical settings or investigators examine in experimental settings or epidemiologic studies, sleep problems in childhood must be viewed in the context of normal physical and cognitive/emotional phenomena that are occurring at different developmental stages. For example, temporary regressions in sleep development often accompany the achievement of motor and cognitive milestones in the first year of life. Similarly, an increase in nighttime fears and night wakings in toddlers may be a temporary manifestation of developmentally normal separation anxiety peaking during that stage. In addition, sleep problems in the pediatric population must be viewed against a backdrop of the normal developmental trajectory across childhood and developmental norms; i.e., ‘‘normal’’ bedtime behavior, time to sleep onset, and sleep duration are obviously dramatically different in a 6-month old, 6-year old, and 16-year old. Finally, because the developmental continuum includes the transition into adulthood, insights into the prevalence, types, and impact of, as well as risk and protective factors for, sleep problems in childhood have direct relevance in regards to an increased understanding of sleep issues at all life stages. Despite the obvious importance of viewing sleep problems in both children and adults from a developmental perspective, the body of literature in developmental aspects of pediatric sleep research at this juncture is still relatively sparse. For example, there are surprisingly little large-scale epidemiological data currently available, which systematically define normal sleep and wakefulness patterns and sleep duration in the pediatric population. Most of the existing studies have utilized subjective, parentreport, retrospective, cross-sectional surveys in selected populations. Although cross-sectional studies yield important information regarding sleep in discrete age groups, by their nature they neither describe the evolution and persistence of sleep/wake patterns over time, nor do they help to elucidate the complex reciprocal relationship between sleep and cognitive/emotional development from the prenatal period through adolescence. There are even more limited data from studies utilizing more ‘‘objective’’ methods of measuring sleep quality and duration, such as polysomnography and actigraphy; and many of these studies were conducted prior to the establishment of accepted sleep monitoring and scoring standards. There are many potential reasons for this relative knowledge gap. First, as with many behavioral issues in childhood, the patient is rarely the one who presents with a chief complaint of a sleep problem. Thus, parental concerns and subjective observations regarding their child’s sleep patterns and behaviors often define sleep disturbances in the research, as well as in the clinical context. Parental recognition and reporting of sleep problems in children also varies across childhood, with parents of infants and toddlers more likely to be aware of sleep concerns than those of school-aged children and adolescents (and to report them to clinicians and researchers). Furthermore, the very definition of what behaviors constitute a sleep ‘‘problem’’ is often highly subjective and is frequently determined by the amount of disruption caused to parents’ sleep. Culturally based values and beliefs regarding the meaning, importance, and role of sleep in daily life, as well as culturally based differences in sleep practices (e.g., sleeping space and environment, solitary sleep vs. co-sleeping, use of transitional objects) also have a profound effect on how a parent