<p>Morningness/Eveningness (M/E) is a relevant characteristic related to adolescent mental and physical health. In this study, the Morningness-Eveningness Scale for Children (MESC) was adapted to a Río de la Plata population, and its psychometric properties were studied in a sample of 368 Uruguayan adolescents attending different school shifts. Scale present good reliability, high temporal stability, good concurrent validity with mid-sleep point, and a two-factor solution. No differences in scores were observed between sexes or age groups. Scores were different between school shifts. This study show that MESC score is a reliable and valid characterization of M/V in adolescents, but social pressures should be considered when using this scale.</p>