Prior research has established associations between sleep duration and body mass index (BMI) scores and risk for obesity in middle childhood, but it is less clear whether other objectively- and subjectively-measured sleep indicators may be associated with BMI scores, weight status (e.g., obesity), and other estimates of weight and body fat such as waist circumference (WC) and percent body fat. Empirical studies have also demonstrated independent associations between broad self-regulation and sleep indicators and BMI scores, but no study to date has tested these factors in a model together and the extent to which associations between normative sleep problems, weight indicators, and effortful control (EC) may be explained by shared genetic or environmental influences. Data from a large longitudinal study of twins was used to test phenotypic associations between sleep problems at eight years and weight indicators at nine years, including whether EC at eight years moderates these associations. Additionally, multiple quantitative behavior genetic models were used to estimate unique and shared genetic and environmental covariances among normative sleep problems, weight indicators, and EC at eight years of age and whether additive genetic influence on weight in middle childhood differs by child weight status group. Phenotypic findings showed that greater sleep duration at eight years predicted greater decreases BMI at nine years of age for children with low levels of EC at eight years. Greater sleep midpoint variability at eight years predicted greater increases in percent body fat from eight to nine years of age for children with low EC at eight years. Behavior genetic findings showed greater environmental influences on parent-reported sleep duration and quality, as well as objective sleep midpoint variability. Similarly, associations between parent-reported sleep duration and sleep midpoint variability and other sleep indicators and EC were primarily accounted for by shared environmental factors. In contrast, there was high additive genetic influence on objective sleep quantity and quality, all weight indicators, and EC. Many of the associations between sleep indicators, sleep and weight indicators, and among weight indicators were entirely accounted for by shared additive genetic factors, suggesting that common, underlying sets of genes explain these relations.

Understanding how interpersonal relationships, such as parenting and sibling relationships, may contribute to early sleep development is important, as early sleep dysregulation has been shown to impact later sleep behavior (Sadeh & Anders, 1993), as well as cognitive and behavioral functioning (Gregory et al., 2006; Soffer-Dudek et al., 2011). In addition, twin studies provide an optimal opportunity to estimate genetic and environmental contributions to parenting, sibling relationships and child sleep, as they are influenced by both genetic and contextual factors. As such, the current thesis examined whether parental punitive discipline and sibling conflict were associated with child sleep duration, dysregulation and daytime sleepiness at 12 months, 30 months, and five years in a longitudinal sample of young twins recruited through birth records (Lemery-Chalfant et al., 2013). Mixed model regression analyses and quantitative behavioral genetic models (univariate and bivariate) were conducted to explore bidirectional relations and estimate genetic and environmental contributions to parental punitive punishment, sibling conflict and child sleep parameters. Sleep duration and dysregulation showed stability over time. Parental punitive discipline did not predict concurrent or future sleep parameters, nor were there bidirectional relations between punitive discipline and child sleep behaviors. Greater sibling conflict at five years was associated with shorter concurrent child sleep duration and greater daytime sleepiness, suggesting that sibling conflict may be a critical interpersonal stressor that negatively impacts child sleep. Shared environmental factors also accounted for the greatest proportion of the covariance between sibling conflict and sleep duration and daytime sleepiness at five years. These findings hold promise for sleep and sibling interaction interventions, including educating parents about fostering positive sibling relations and teaching caregivers to utilize specific parenting behaviors that may encourage better child sleep behaviors (e.g., establishing bedtime routines). Future studies should aim to understand the nuances of associations between family relationships (like punitive discipline and sibling conflict) and child sleep, as well as other explore person- and family-level factors, such as child negative emotions and parenting, that may influence associations between family relationships and child sleep.