Longitudinal interplay among body mass index, sleep duration, and phone dependency in adolescents: data from six waves of two nationally representative cohorts

Mobile phone dependency, sleep duration, and body mass index (BMI) are interconnected facets of an adolescent's lifestyle. Existing research has largely yet explored only two of these variables in tandem at between-person levels. This study addresses this gap by examining the temporal dynamics among them over six timepoints, at both between- and within-adolescent levels. Annual data (N = 4,729, 48.8% female) from two nationally representative South Korean birth cohorts were utilized, covering Grade 5 through Grade 10 and Grade 8 through post-high school. Both a cross-lagged panel model (CLPM) and a random-intercept CLPM indicated that phone dependency had inconsistent effects on sleep duration, while having minimal effects on BMI. While longer sleep duration is linked to a lower BMI, a lower BMI is associated with increased phone dependency at inter-adolescent levels. Longer sleep durations lead to greater phone dependency, both at inter- and intra-adolescent levels during educational transitions, suggesting the need for increased support during these critical periods. The temporal relations among mobile phone dependency, BMI, and sleep duration are intricately intertwined, emphasizing the importance of considering developmental periods and gender differences among adolescents.