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 <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.

No abstract

Dim light melatonin onset (DLMO) is the most reliable measure of human central circadian timing. Its modulation by light exposure and chronotype has been scarcely approached. We evaluated the impact of light changes on the interaction between melatonin, sleep, and chronotype in university students (n = 12) between the Antarctic summer (10 days) and the autumn equinox in Montevideo, Uruguay (10 days). Circadian preferences were tested by validated questionnaires. A Morningness–Eveningness Questionnaire average value (47 ± 8.01) was used to separate late and early participants. Daylight exposure (measured by actimetry) was significantly higher in Antarctica versus Montevideo in both sensitive time windows (the morning phase-advancing and the evening phase-delaying). Melatonin was measured in hourly saliva samples (18–24 h) collected in dim light conditions (<30 lx) during the last night of each study period. Early and late participants were exposed to similar amounts of light in both sites and time windows, but only early participants were significantly more exposed during the late evening in Antarctica. Late participants advanced their DLMO with no changes in sleep onset time in Antarctica, while early participants delayed their DLMO and sleep onset time. This different susceptibility to respond to light may be explained by a subtle difference in evening light exposure between chronotypes.

Morningness-Eveningness (M-E) has been associated with school performance when school activities are arranged early in the morning. This study aims to evaluate this association in 224 adolescents of a secondary school in Montevideo, Uruguay, attending either morning or afternoon shifts. Students' socio-demographic characteristics and performance indicators were similar across shifts, while afternoon-shift students exhibited later circadian preferences. After controlling for socio-demographic and other educational variables, eveningness was associated with lower grades only in morning-shift students. Our study suggests that eveningness influence on school performance is contingent on the temporal arrangement of scholar activities.

Annual fishes inhabit temporary ponds that dry out seasonally and the adaptations to survive this extreme condition include high metabolic rates and an elaborate courtship behavior which ends in the deposition of drought-resistant eggs, capable of going through diapause stages in the substrate. The pronounced sexual dimorphism that these fishes show suggests that sexual selection could play a key role in the differentiation, speciation and evolution of this diverse group of fishes. However, there is scarce information about their reproductive isolation mechanisms, including detailed analysis of courtship signals. Herein, we analyzed, qualitatively and quantitatively, the reproductive behavior of Austrolebias reicherti. The behavioral units recognized in this study correspond with the previous analysis of other species of annual fishes. The most frequent unit in males was the lateral display where specific morphological and coloration patterns are displayed to the female. The female's high relative frequency and time of quietness suggest that this unit may have an evaluation role during courtship. In addition to visual displays during courtship, males perform vibrations of the dorsal and anal fins as well as body undulations; these indicate that mechanical signals may be important for attracting females. Our results support the hypothesis of multimodal signals. The conservation of behavioral patterns in courtship displays within Austrolebias suggests that species-level recognition and the barrier to hybridization may not occur at this stage, unless quantitative differences can be identified.Os peixes anuais habitam charcos temporários que secam sazonalmente, e as adaptações para sobreviver a estas condições extremas incluem altas taxas metabólicas e um elaborado comportamento de cortejo, que culmina com a deposição de ovos resistentes à dessecação dentro do substrato, os quais são capazes de atravessar estádios de diapausa. O pronunciado dimorfismo sexual apresentado por estes peixes indica que a seleção sexual poderia estar exercendo um papel chave na diferenciação, especiação e evolução deste grupo diverso de peixes. Entretanto, informações acerca dos mecanismos de isolamento reprodutivo, incluindo uma análise detalhada dos sinais de cortejo, são escassas. Neste artigo é analisado o comportamento reprodutivo de Austrolebias reicherti de maneira qualitativa e quantitativa. As unidades comportamentais utilizadas neste estudo correspondem àquelas incluídas em análises anteriores de outras espécies de peixes anuais. A unidade comportamental mais freqüentemente observada em machos foi a "exibição lateral", onde características espécie-específicas da morfologia e colorido são exibidas para a fêmea. A alta proporção de ocorrência (em freqüência e tempo) do comportamento de "quietude" nas fêmeas indica que o mesmo poderia desempenhar um papel avaliativo durante o cortejo. Outros sinais além dos visuais estão envolvidos no cortejo dos peixes anuais; os machos vibram as nadadeiras dorsal e anal e ondulam o corpo, o que i...