Light Exposure and Children's Sleep: A Twin Study with Objective Measurement

Sufficient sleep in childhood is fundamental to proper development as well as preventing behavioral or emotional complications later in adulthood (Gregory & Sadeh, 2012; Bruni, 2010). Sleep is controlled by a 24-hour cycle of hormonal regulation termed the circadian rhythm, which is controlled by different environmental inputs such as light (Reppert & Weaver, 2002). Previous research has also demonstrated that light exposure at night can delay the night phase production of specific hormones that promote sleep (Zeitzer, Dijk, Kronauer, Brown, & Czeisler, 2004; Chang, Aeschbach, Duffy, & Czeisler, 2015), which in turn delays sleep onset. Such studies involving the effects that light may have on sleep have focused on adult subjects, however, and it is important to explore this idea in childhood to promote proper development. The first aim of this study was to examine the effects of light exposure in the hour before bedtime on different measures of sleep in middle childhood. The second aim was to determine the genetic and environmental contributions to light exposure and sleep. A diverse sample of 490 twin children was assessed at 8 years of age. Twins followed a week long protocol in which they wore actigraph watches that collected data on both light and sleep. Zero-order correlations with subsequent multilevel regression analyses showed that any light exposure in the hour before bedtime was significantly positively associated with sleep onset latency. Twin intraclass correlations indicated no heritability for light exposure, but did indicate some heritability ranging from 7-66% for the sleep indicators. Overall, these findings regarding the impacts of sleep in childhood build upon an area of research that has only been explored in adulthood. These impacts of light on sleep in childhood suggest that possible interventions ought to be explored for implementation to minimize the long-term effects of altered sleeping patterns in childhood.