A better alignment between chronotype and school timing is associated with lower grade retention in adolescents

Humans exhibit circadian or close to 24 h rhythms in their physiology and behavior, which differ between individuals. These differences are captured by chronotype, which is the expression of each individuals’ endogenous circadian timing under specific conditions (including the light-dark cycle)1,2, and ranges on a continuum between early and late types. Chronotype has a genetic basis3,4,5, but it is also modulated by multiple factors, such as light exposure1,6, age7,8 and cultural and social cues6,9,10. It can be assessed by evaluating behavioral11,12,13 and/or physiological rhythms14,15,16 as well as with standardized questionnaires. One of the most widely used questionnaires is the Munich chronotype questionnaire (MCTQ)17, from which the sleep-corrected Midpoint of Sleep on Free days (MSFsc) is obtained. MSFsc is a chronotype proxy based on sleep timing and it highly correlates18,19 with the phase of sleep-rest activity11,12,13,17,20 (evaluated with sleep diaries and actigraphy) and endogenous physiological rhythms14,15,16,21,22,23. Thus, this questionnaire is an easy and reliable method to assess chronotype.

Secondary school starts very early in the morning for most students around the globe. Students´ chronotypes at this age become progressively delayed reaching a peak of lateness at the end of adolescence7,8,24. This misalignment between biological timing (i.e. chronotype) and social obligations (i.e. school schedule) is proposed to be the main cause why many adolescents present chronic sleep deprivation and social jetlag (i.e. discrepancy of sleep timing between free days and weekdays), which in turn, show to be associated with health problems and impaired cognitive performance25,26,27,28,29,30,31. Several interventions delaying school start time lead to an improvement of adolescents’ mood, wellbeing32,33,34 and academic performance35,36. Although not conclusive37, these results suggest that a better alignment between adolescents’ internal timing and school schedules could be beneficial to improve adolescents’ academic performance.

Although adolescents exhibit later chronotypes than children and adults, there is a large intrinsic variability in their chronotypes7,8. Some studies show that students with earlier chronotypes attending school in the morning perform better than their peers with later chronotypes38,39,40. However, it is not clear whether this result occurs because early chronotypes perform better than late chronotypes (‘chronotype effect’) or because early chronotypes, unlike late chronotypes, are being evaluated at their best time of the day (‘synchrony effect’). Evidence of the synchrony effect was found in executive functioning41,42,43, priming44, memory45, and fluid (but not crystallized) intelligence46,47. At school, results vary according to which school subjects are considered. Morning-attending students perform better in math and chemistry if they present an early chronotype. However, this effect is smaller or absent for native language and geography48,49. Therefore, the chronotype and/or the synchrony effects might differentially affect performance depending on school subjects. Finally, because chronotype progressively delays during adolescence, younger students are expected to be less affected than older ones.

There are only a handful of studies comparing how academic performance (measured as school grades, hereafter used as synonyms) is affected by chronotype in morning and afternoon school timings. These studies showed that adolescents with earlier chronotypes perform better in the morning school timing, but not in the afternoon, where academic performance does not vary across chronotypes50,51,52,53,54,55. This seems to imply that chronotype is not the only factor affecting adolescents’ academic performance, but this conclusion cannot be established because, in these experiments, students were not randomly assigned to school timings. Thus, results can be bias because students’ preferences and baseline differences in chronotype and academic performance between school timings. In addition, these results are compatible with a pure effect of synchrony which could be masked if the afternoon school timing is yet too early for late chronotypes to perform better than early chronotypes. Hence these studies cannot rule out between these different scenarios explaining how the interaction between chronotype and school timing affect adolescents’ grades: (1) variations in academic performance are completely explained by the interaction between chronotype and school timing, with higher grades associated with a better alignment between school schedules and students’ chronotype (i.e. synchrony effect); (2) variations in academic performance are completely explained by chronotype, with earlier chronotypes obtaining higher grades than later chronotypes (i.e. chronotype effect); (3) both the chronotype and its interaction with school timing modulate academic performance; as a result, earlier chronotypes perform better, but the magnitude of this association will be related to how well chronotype and school timing are aligned (i.e. both chronotype and synchrony effects)56.

Recently, in a cross-sectional study of our group we try to disambiguate these possible scenarios addressing the mentioned confounds (lack of random assignment and lack of an evening school timing). We investigated performance in a natural educational setup where students in their first year were randomly assigned to one of three different school timings: morning (07:45–12:05), afternoon (12:40–17:00), or evening (17:20–21:40)56. This random assignation suggests no bias for factors that can condition school timing assignment, such as socio-economic status, chronotype preferences or previous academic achievement51. The study showed that, for morning-attending students, early chronotypes performed better than late chronotypes in all school subjects and, particularly, in math. On the other hand, this effect was not observed in any school subject for students who attended school in the afternoon. Finally, older students with late chronotypes benefit from evening classes, especially on native language56.

Here, we present a longitudinal study, further capitalizing this unique educational setup. We evaluated students in their 1st (13–14 years old) and 5th (17–18 years old) year of secondary school56,57. This longitudinal design allows us to assume that the differences observed between 1st and 5th year in students’ academic performance are not due to interindividual variability but due to age-related changes. Moreover, chronotype interacting with school schedule could not only modulate student academic performance (i.e. grades), but also more global measures of school success such as grade retention (i.e., the proportion of students that repeat at least one school year throughout secondary school). In the last few years it was reported that short sleep duration predicts class retention in college students58,59, but the effect of chronotype on grade retention in adolescents remains unknown. Our longitudinal study providing data of which students that start their 1st year in 2015 do not reach 5th year four years later, allowed us to address this gap in knowledge.

The aim of this work is to understand whether, and how, chronotype interacting with school timing affects academic success, measured as both academic performance (i.e. school grades) and grade retention (i.e. repeat a year). The impact of chronotype and school timing in academic success was addressed in three different ways. First, we test whether academic performance differs between school timings in 1st and 5th year: a better performance in one of the three school timings could indicate a better alignment between school schedules and students’ internal timing. As Argentinian adolescents’ present particularly late chronotypes57, we hypothesize that students attending afternoon and/or evening school timings will present better academic performance than those attending school in the morning (synchrony effect). Second, we study how synchrony and chronotype effects can modulate academic performance considering interindividual differences in adolescents’ chronotype. Taking into consideration our previous cross-sectional results56, here we hypothesize that both synchrony and chronotype effects will act together to modulate academic performance. Third and importantly, we study whether chronotype interacting with school timing predicts grade retention. We hypothesize that, even controlling for academic performance, students with later chronotypes would present higher odds of experiencing grade retention (i.e. not reaching their last school year) when attending morning school timing; and that this effect would be gradually lower for later school timings.

Altogether, here we study whether and how chronotype alone and/or including its alignment with the school schedule affect academic success.