The phase delay on the sleep-wake cycle resulting from biopsychosocial changes related with adolescence is exacerbated by the level of urbanization. However, early school start times conflict with this delay, leading to several negative consequences. This study aims to compare sleep habits and quality, daytime sleepiness, and attention between adolescents from morning and afternoon shifts across different urbanization levels. A total of 159 Brazilian adolescents from technical high school (15.3 ± 0.8 years old; 101 girls/58 boys) completed the following questionnaires: "Health and Sleep," the Pittsburgh Sleep Quality Index, and the Pediatric Daytime Sleepiness Scale, in addition to a 10-d sleep diary that included the Maldonado Sleepiness Scale. Attention was evaluated by a continuous performance task. Sleep and attention were compared between shifts (Morning: n = 78/Afternoon: n = 81) and degrees of urbanization (High: n = 97/Low: n = 62) using MANCOVA. Independent of urbanization level, morning-shift students woke-up earlier, had a shorter time in bed at night and over a 24-h period (nocturnal sleep + nap), showing higher irregularities in wake-up times, along with higher sleep debt at night and over 24-h, social jetlag, daytime sleepiness, sleepiness upon wake-up time, frequency of naps, and non-spontaneous waking during the week than those in the afternoon shift. However, there were no differences in attention performance. Independent of shift, students from higher urbanization regions showed later bed and wake-up times and a shorter time in bed at night and over 24 h on weekends, and higher sleepiness upon wake-up time on weekdays. Despite longer reaction time in phasic alertness and selective attention, they showed a lower percentage of wrong responses in tonic and phasic alertness, selective attention, and a higher time‑on‑task stability. Thus, early school starting time is a strong modulator for adolescents' sleep-wake cycle regardless of the urbanization level. Further studies are necessary to analyze the relationship between attention and school shift.
