As adolescents get older, they become more evening oriented and, because they are usually expected to wake early to attend school, they often present daytime sleepiness, which is associated with negative outcomes. It is still unclear if this is similar cross-culturally. Here, we studied morningness-eveningness and daytime sleepiness in early adolescence from two different developing nations (Brazil and Iran). A total sample of 697 Iranian and Brazilian early adolescents (9- to 15-year-old; 358 boys) from Tehran, Iran, and São Paulo, Brazil, varying in age and parental schooling (a proxy of socioeconomic status: SES) completed the Morningness–Eveningness Scale for Children (MESC) and the Pediatric Daytime Sleepiness Scale (PDSS) and reported their total sleep time on school nights. They also filled in the Pubertal Developmental Scale to determine their pubertal status. A negligible cross-cultural difference in morningness-eveningness was found, indicating that Brazilians showed a slight circadian-phase delay compared with Iranians throughout all tested ages. There was also seen a very slight increase in phase delay as early adolescents aged, indicative of more eveningness. However, there were no country differences in daytime sleepiness once total sleep time during school nights was controlled for, which was the only factor that affects PDSS scores.
