Infant Sleep Quality Predicts Changes in Temperament Across the First Year of Life

Sleep and temperament are central to early development and are linked to later socioemotional outcomes. Although sleep problems are often associated with difficult temperament, most prior studies rely on cross-sectional designs and parent reports, leaving directionality unclear. The aim of this study was to examine bidirectional longitudinal associations between infant sleep and temperament across the first year, with an emphasis on within-person effects. We recruited 232 couples during pregnancy. Assessments were conducted when infants were 4, 8, and 12 months old. Infant sleep was assessed over seven nights at each wave using actigraphy and sleep diaries. Sleep duration and wake after sleep onset (WASO) were derived from both methods, and sleep diaries additionally provided a measure of perceived sleep quality. Temperament was assessed via maternal report using the Infant Characteristics Questionnaire (Fussy and Unpredictable subscales). Random Intercept Cross-Lagged Panel Models (RI-CLPMs) were used to separate within- from between-person effects. Poorer reported sleep quality and higher WASO were concurrently associated with greater fussiness and unpredictability, whereas actigraphy-based measures were not. RI-CLPMs revealed significant within-person concurrent associations, particularly at 8 months. Within-person cross-lagged effects emerged from sleep to temperament, but not vice versa: poorer sleep quality and greater WASO predicted subsequent increases in unpredictability. No significant between-person associations were observed. The findings demonstrate that disruptions in infant reported sleep quality predict later increases in unpredictability, highlighting temporal associations between sleep and emerging temperament. Future research is needed to determine if interventions targeting infant sleep support early behavioral development. • Bidirectional links between infant sleep and temperament were tested longitudinally • RI-CLPMs were used to separate within-person from between-person effects • Poorer within-person infant sleep quality predicted increases in unpredictability. • Infant temperament did not predict sleep quality • Findings suggest infant sleep may be a driver of early temperamental regulation.