Short Sleep Duration and Extremely Delayed Chronotypes in Uruguayan Youth: The Role of School Start Times and Social Constraints

Abstract

<jats:p> During adolescence, biological, psychosocial, and contextual factors converge in a “perfect storm” and have been put forward to explain the delay in chronotype observed at this age and the prevalence of disrupted sleep. This study provides evidence to support that chronotype and sleep patterns (particularly sleep duration) are socially constrained and to identify novel significant social predictors. Uruguayan public school activities are arranged in up to 4 shifts, creating a natural experiment to examine the effect of school timing on questionnaire-based assessments of sleep and chronotype. In this study, 268 high school students (15-18 years old) who attended school either on morning (0730 to 1130 h) or afternoon shifts (1130 h to 1530 h) responded to an adapted School Sleep Habits Survey. Students attending afternoon shifts had later chronotypes (a 1.5-h later midpoint of sleep on free days adjusted for sleep debt) than those attending the morning shift. Besides shift, evening social activities (including dinner time) were further identified as key predictors of late chronotypes, whereas age and gender were not. Sleep on school days was overall advanced and reduced with respect to weekends, and these effects were stronger in morning-shift students. Weekend sleep duration was similar between shifts, which probably caused the prevalence of reduced sleep durations (average weekly sleep duration, SDweek &lt;8 h) to be higher in morning-shift students (almost 80%) than in afternoon-shift ones (34%). Reduced sleep duration was significantly higher in morning-shift students. In addition, age, chronotype, and dinner time became relevant determinants of sleep deficit only in the morning-shift students. Besides the important social constraint of early school start time, this is the first study to confirm the significance of other types of social pressures on both adolescents’ chronotype and sleep deficit, which can be useful as potential new targets for effective policies to protect adolescent sleep. </jats:p>

Sleepiness is a signal to go to bed: data and model simulations

“…From the point of view of the regulation of sleep by homeostatic and circadian factors [ 32 , 33 ], as in the two-process model, we expect low levels of evening sleepiness not only to lead to later bedtimes, but also, because of the circadian regulation of sleep timing and duration [ 32 , 33 ], we expect later bedtimes to be associated with shorter sleep durations. Later bedtimes should also lead to shorter sleep durations due to social constraints such as the need to get up for work or school [ 34 ]. Short sleep duration will lead to higher levels of sleepiness in the morning and continue to be elevated until the next evening.…”

One year later: longitudinal effects of flexible school start times on teenage sleep and subjective psychological outcomes

“…Other studies found similar effects only when school started much later, such as in the afternoon [61,62].…”

“…Double-blinding, the gold standard in terms of evidence level, is, of course, inherently unfeasible in this context, and it seems almost impossible to convince schools to participate in randomization [36]. Although there are some real-life settings, such as in Uruguay or Argentina, where students are randomly assigned to morning, middle, and afternoon school shifts [37,38], this is not the case in most other countries around the world. The few longitudinal studies that exist often covered ≤6 months in their follow-ups [30] (but see [39][40][41][42][43]), and are thus prone to seasonal confounding.…”

Are flexible school start times associated with higher academic grades? A 4-year longitudinal study

“…In conclusion, we highlight that current early school start times around the globe are detrimental for sleep and health and likely do not allow students to excel as much as they could. Many studies have shown positive effects on sleep or well-being, when school starts were delayed [21][22][23][24] or in systems were school already starts much later, such as in Uruguay or Argentina [76,77]. Thus, it seems fair to argue that later starts are beneficial for students in terms of health and well-being.…”