Recent reviews raised the idea of a bidirectional relation between sleep behavior and affect in adults, but little is known about this interplay in general and especially regarding children. In this micro-longitudinal study, the interplay of sleep and affect was captured directly in children's daily life context in and out of school through ambulatory assessment. For 31 consecutive days, 110 elementary school children (8-11 years old) provided information about their last night's sleep and reported their current affect at four daily occasions in school and at home on smartphones. A multilevel approach was used to analyze the relation between sleep and affect the next day (morning, noon, and afternoon) and the relation between evening affect and subsequent sleep. At the within-person level, sleep quality was related to all observed facets of affect the next day and the strongest effects were found in the morning. The effect of sleep quality on positive affect was particularly pronounced for children who on average went to bed early and slept long. There were, however, no direct within-person effects of sleep quantity on affect. Furthermore, evening affect was related to subsequent sleep. The findings support the idea of a bidirectional relation between affect and sleep in children's daily life (including school). They suggest that good sleep provides a basis and resource for children's affective well-being the next day and demonstrate the importance of analyzing within-person variations of children's sleep. Micro-longitudinal findings can contribute to explain how macro-longitudinal relations between sleep and affect develop over time.
Keywords: Children; Experience sampling; Intraindividual variability; Mood; Sleep; Within-person.
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