OBJECTIVES: We searched for interactions between PER3 gene VNTR polymorphism, latitude, sleep duration, diurnal sleepiness, and social jetlag. DESIGN: We selected samples from 3 distinct cities along the latitudinal range of Brazil and comprising the same time zone. SETTING: Undergraduate universities located in 3 major cities of Brazil. PARTICIPANTS: A total of 980 undergraduate students: 276 from Maceio (latitude 9°), 358 from Campinas (latitude 22°), and 346 from Porto Alegre (latitude 30°). MEASUREMENTS: PER3 variable number of tandem repeats genotyping, diurnal sleepiness, sleep duration (weekdays and weekend), chronotype, and social jetlag. RESULTS: Latitude is associated with a differential expression of circadian and sleep profiles. We observed a shift toward eveningness with increased latitude and increased social jetlag and diurnal sleepiness at latitude 30°. Moreover, our results suggest that the PER3 variable number of tandem repeats polymorphism has a modulatory effect on these circadian and sleep profiles: the variant PER3 is associated with a smaller difference in the sleep duration on weekdays among different latitudes and is associated with longer sleep duration on weekends just at latitude 30°, even when compared to both other genotypes at the same latitude. On the other hand, irrespective of the genotype, volunteers from latitude 30° expressed increased social jetlag and diurnal sleepiness. CONCLUSIONS: The seasonal variation in the light/dark cycle, tied to latitude, together with the tight social time constraints that young adults are subjected to during weekdays, generates differences in the sleep phenotypes. Volunteers with the PER3 variant who live farther from the equator have a greater increase in their weekend sleep duration.