OBJECTIVES: The transition from flexible preschool schedules to fixed early primary school start-times may lead to children adopting earlier sleep-wake schedules. However, it is unknown whether this transition reflects shifts in chronotype, or how chronotype changes contribute to sleep problems. Using longitudinal data, we investigated the stability of chronotype during this transitional period and how chronotype changes associate with sleep problems.
METHODS: Data were analyzed for 256 children in the Growing Up in Singapore Towards healthy Outcomes birth-cohort study. Caregivers reported children's bedtimes and waketimes, and completed the Morningness-Eveningess (M/E) scale from the Children's Chronotype Questionnaire when children were 4.5 and 8 years old. Time-in-Bed midpoint (TIB-Midpoint) was derived as the midpoint between bedtime and waketime. Sleep problems at 8 years old were measured using the caregiver-reported Children's Sleep Habits Questionnaire. Paired-samples t-tests compared bedtimes, waketimes, TIB-midpoint and M/E scores between timepoints. Chronotype stability was evaluated through Pearson's correlation and intraclass correlation coefficient (ICC). Linear regressions examined associations between changes in M/E scores and sleep problems.
RESULTS: M/E scores at both timepoints were moderately correlated (r = .60, p < .001), with an ICC (.59) suggesting moderately stable individual differences. Chronotype showed an overall shift towards morningness at school-age (t(255) = 5.05, p < .001), supported by earlier bedtimes, waketimes and TIB-midpoints (all p < .05). Individual shifts towards eveningness were linked to daytime sleepiness and total sleep problems (both p < .010).
CONCLUSION: Changes in scheduling demands during the preschool-to-school transition parallel an overall shift towards morningness, alongside moderately stable individual differences in chronotype. Understanding chronotype stability may inform risk-profiling and interventions for sleep problems.
