Understanding Circadian Physiology in Early Childhood: the Role of Napping and Light at Night

The circadian clock, localized to the suprachiasmatic nucleus of the anterior hypothalamus, is responsible for regulating rhythms in behavior and physiology including the timing and secretion of the hormone melatonin. Light is the strongest environmental input to the circadian system. Specifically, light at night can delay the timing of the melatonin rhythm and suppress secretion of this sleep-promoting hormone. Although data on factors that influence the biological clock of adolescents and adults is expanding, very little is known about circadian physiology in the early childhood years. The collective aim of this dissertation was to assess the influence of several modifiable factors on the circadian physiology of young children. In study 1, sleep and circadian timing were compared between napping and non-napping toddlers. Napping toddlers had significantly later bedtimes, sleep onset times and circadian phases than non-napping toddlers. These differences in circadian timing are likely mediated by the later bedtimes of nappers, which facilitate light exposure later in the evening thereby delaying the clock. The purpose of study 2 was to quantify the magnitude of light-induced melatonin suppression in response to evening light exposure in preschoolers. Children experienced ~90% melatonin suppression in response to a 1 hour long bright light stimulus before bedtime, an effect that persisted up to 50 minutes after the light stimulus was terminated. In study 3, we examined children’s evening light exposure in association with circadian timing. The amount of light children were exposed to during the 2 hours before bedtime predicted variance in circadian timing over and above bedtime alone in our sample of young children. The findings of this dissertation demonstrate the robust sensitivity of the circadian system of young children to light at night and represent important first steps in understanding fundamental aspects of circadian physiology during the early years of life