The research question centers on whether the association between later chronotypes and lower academic performance is a direct effect of chronotype or a consequence of school starting too early for late chronotypes. The study's purpose is to clarify this by examining how school timing and chronotype interact to affect grade retention, a more comprehensive measure of academic success than grades alone. This is important because early school start times are prevalent globally, often conflicting with adolescents' naturally later chronotypes, potentially leading to sleep deprivation, social jetlag, and impaired cognitive performance. Previous research has shown an association between chronotype and academic performance, but it's unclear if this is due to the inherent differences in performance between chronotypes (chronotype effect) or the mismatch between chronotype and school schedule (synchrony effect). The study uses a unique sample of students randomly assigned to different school start times to control for confounding variables, addressing limitations in previous research.

Existing literature shows a correlation between later chronotypes and lower academic performance. However, studies examining the interaction between chronotype and school timing are limited and have methodological flaws, such as lacking random assignment to school timings. Some studies suggest that early chronotypes perform better in the morning, but it is not definitively determined whether this is due to a chronotype effect or a synchrony effect. The impact of chronotype and school timing on academic performance might vary depending on the subject, with math and science potentially more sensitive to this interaction than language-based subjects. Studies focusing on academic performance (grades) might be insufficient, as chronotype may influence broader measures of success, like grade retention. The authors' prior cross-sectional study provided a foundation for this longitudinal work, demonstrating that the random assignment of students to different school timings in a natural setting could be used to disentangle the chronotype and synchrony effects.

This longitudinal study took place at a secondary school in Buenos Aires, Argentina, where students were randomly assigned to one of three school timings: morning (7:45 AM), afternoon (12:40 PM), or evening (5:20 PM). Data was collected in two phases (June 2015 and July 2019) from students in their first and fifth years of secondary school. Chronotype was assessed using the Munich Chronotype Questionnaire (MCTQ), yielding the sleep-corrected Midpoint of Sleep on Free days (MSFsc) as a measure. Academic performance was measured using grades in various subjects, categorized into math, language, and other subjects. Grade retention was determined by tracking which first-year students reached their fifth year. Linear mixed-effects models were used to analyze the relationship between chronotype, school timing, age, school subject, and academic performance, controlling for gender. Logistic regression models were used to analyze the predictors of grade retention, including grades, chronotype, and the interaction of chronotype with school timing. The study controlled for gender and accounted for potential confounding variables such as classroom and grade type (general or integrative). Odd ratios were calculated to understand the relationships between variables and grade retention, taking into account interactions between chronotype, school timing, and grades.

The study found a significant synchrony effect on academic performance. In their first year, afternoon-attending students performed better in math than evening-attending students, while in their fifth year, evening-attending students excelled in math. This suggests improved alignment between biological rhythms and school timing as students age. A chronotype effect was also observed, with earlier chronotypes consistently performing better, particularly in math, and this effect was most pronounced in older students. The magnitude of the chronotype effect varied by school timing, with the effect strongest in the morning, indicating the presence of a synchrony effect that partially counteracts the chronotype effect. In terms of grade retention, the analysis revealed a significant interaction between chronotype and school timing. Later chronotypes showed higher odds of grade retention in the morning than in the evening, even after controlling for academic performance. This highlights the importance of synchrony between chronotype and school timing in minimizing grade retention. Students with lower math grades in the first year were more likely to experience grade retention, especially if their chronotype was later than average. Lower language grades were also associated with grade retention, particularly in the morning. The odds ratio analysis further substantiated these findings. Notably, there was evidence that morning-attending students with lower language grades might have a higher rate of grade retention than those in other school timings, highlighting the importance of the interaction between these factors.

The findings address the research question by demonstrating that both chronotype and its alignment with school timing (synchrony) significantly affect academic success. While a chronotype effect (inherent difference in performance between chronotypes) exists, particularly in math performance, especially for older students, the synchrony effect is more evident in the impact on grade retention. The results suggest that math performance is more strongly influenced by chronotype and its interaction with school timing than language. This may be because math relies more heavily on fluid intelligence, known to be influenced by the chronotype and synchrony effects. The evening school timing showed relatively less negative impact on the chronotype effect compared with the morning and afternoon school timings, which supports the idea that synchrony effects may mitigate the disadvantage faced by late chronotypes. Grade retention appears to be influenced by the combined effects of chronotype and school timing, reinforcing the need to consider the interplay of these factors when assessing academic success.

This longitudinal study reveals that both chronotype and its synchrony with school timing significantly impact adolescent academic success, measured by both academic performance and grade retention. The results support the implementation of policy changes, such as later school start times, that better align with adolescent chronotypes, particularly for older students and those with later chronotypes. This research highlights the need for educators, policymakers, and the chronobiology community to collaborate in designing educational policies that promote adolescent wellbeing and academic success.

The study's limitations include the use of teacher-assigned grades, which may not be entirely objective. Chronotype was assessed via self-report questionnaires; objective measures like actigraphy could strengthen future studies. The study uses correlational data; causal relationships cannot be definitively established. Finally, the data might not fully distinguish between grade retention and other reasons for not completing secondary school in the expected timeframe, though the selective nature of the school mitigates this limitation.