Adolescence is a critical period for both the development of overweight/obesity and the transition towards a later chronotype. This shift is often accompanied by an increase in social jetlag, the discrepancy between the individual's biological clock and socially imposed sleep-wake schedules. Existing literature shows a correlation between late chronotype, social jetlag, and higher BMI in adolescents. However, most studies are cross-sectional, limiting the understanding of the causal relationship and the dynamic interplay between chronobiological and body composition changes over time. This study aimed to address this gap by examining the longitudinal associations between changes in chronotype, social jetlag, and changes in body composition measures (BMI-SDS, FMI, FFMI) during adolescence. The researchers hypothesized that shifts towards a later chronotype or increased social jetlag would be associated with unfavorable changes in body composition, potentially varying across different age groups within adolescence. Understanding these associations is crucial for developing effective interventions to mitigate the risk of obesity and related health problems in adolescents.

Numerous studies have linked chronotype and social jetlag to body mass index (BMI) and other body composition measures. Studies have found that adolescents with later chronotypes tend to have higher BMIs. Moreover, social jetlag, the difference between weekday and weekend sleep timing, has also been associated with increased BMI and adiposity. However, most of this evidence is based on cross-sectional studies, limiting the ability to draw causal inferences. Longitudinal studies are needed to understand the temporal relationship between chronobiological shifts and changes in body composition. The existing literature identifies the age of 12 as a potential turning point in the development of eveningness, with a peak in lateness around age 16, although these values may vary across populations. The adolescent period is also a time of significant hormonal changes and rapid growth, making it a crucial period to study the interplay of chronobiology and body composition. This research sought to utilize a longitudinal dataset to assess the dynamic relationship of these factors.

This study utilized data from the DONALD (Dortmund Nutritional and Anthropometric Longitudinally Designed) open cohort study, a prospective study following participants from infancy to adulthood. The analysis focused on adolescents (9-18 years) with at least two measurements of chronotype and anthropometry (N=572 questionnaires from 213 adolescents, 45% female). Chronotype was assessed using the Munich Chronotype Questionnaire (MCTQ), calculating the midpoint of sleep on free days (MSFsc), corrected for sleep debt. Social jetlag (SJL) was determined as the difference between the midpoint of sleep on weekdays and weekends. Anthropometric measurements included height, weight, and skinfold thickness, used to calculate BMI-SDS (using German national reference data), % body fat, FMI (fat mass/m²), and FFMI (fat-free mass/m²). Linear mixed-effect regression models were employed to analyze the associations between changes in MSFsc and SJL and concurrent changes in BMI-SDS, FMI, and FFMI. The models adjusted for covariates including age at baseline, sex, time between measurements, age at take-off (ATO), number of persons in the household, and maternal BMI. The analyses were further stratified by three age groups (<12 years, ≥12 to ≤15 years, >15 years) to investigate age-specific associations. Multiple imputation was used to handle missing data. Sensitivity analyses were conducted to assess the robustness of the findings.

The median follow-up was 2.1 years. Overall, a shift towards a later chronotype (increased ΔMSFsc) was significantly associated with an increase in FMI (B: 0.05, 95% CI: 0.01–0.08). A 1-hour increase in social jetlag predicted an increase in BMI-SDS by 0.08 SDS units (95% CI: 0.01–0.14) and an increase in FMI by 0.04 kg/m² (95% CI: 0.003–0.08). Importantly, these associations were substantially stronger in the age group of 12–15 years (p for interaction <0.001). No significant relationship was observed between changes in chronotype or social jetlag and changes in FFMI. In the 12-15 year old age group, a one hour increase in MSFsc was associated with a 0.11 increase in log10 transformed FMI (p<0.001) and a 0.17 increase in BMI-SDS (p=0.03). A one hour increase in social jetlag was associated with a 0.12 increase in log10 transformed FMI (p=0.001) and a 0.17 increase in BMI-SDS (p=0.003). The changes in MSFsc, SJL, BMI-SDS, FFMI, and FMI were greatest for the age group ≥12 to ≤15 years.

This study provides longitudinal evidence supporting the association between chronotype and social jetlag and changes in body composition, particularly fat mass, during adolescence. The stronger associations observed in the 12–15-year-old age group highlight a potential critical period for intervention. These findings align with previous research demonstrating links between late chronotypes and increased BMI, but extend this knowledge by demonstrating a dynamic relationship over time, specifically focusing on the changes in chronotype and its effects on body composition changes. The lack of association with FFMI suggests that the impact of chronobiological shifts may be predominantly on fat mass rather than lean mass. Several potential mechanisms could underlie these findings, including altered eating patterns, sleep duration, hormonal imbalances, and metabolic changes associated with circadian misalignment. The pubertal period may also play a role due to hormonal influences on circadian rhythms. The study's findings have implications for public health strategies aimed at preventing obesity in adolescents. Early identification of chronotype preferences and interventions to mitigate social jetlag could be beneficial.

This longitudinal study confirms the association between changes in chronotype and social jetlag and alterations in body composition, especially fat mass, during adolescence, with a particular emphasis on the 12-15 year age range. These findings underscore the importance of considering chronobiological factors in obesity prevention strategies for adolescents. Future research should explore the causal mechanisms involved, evaluate the effectiveness of interventions targeting sleep timing and social jetlag, and investigate the long-term consequences of these associations into adulthood.

The study's generalizability might be limited due to its focus on a relatively homogeneous sample of German adolescents with high socioeconomic status. While the prevalence of overweight in the sample is comparable to national averages, differences in lifestyle and environmental factors could influence the results. The use of estimated body fat percentage based on skinfold thickness may introduce measurement error, potentially underestimating the true associations. Although multiple imputation was used, residual confounding from unmeasured or poorly measured variables cannot be entirely ruled out.