Adolescents tend to be much later chronotypes than other age groups. This circadian phase delay is attributed as much to biological as psychosocial factors. Because the consequences of this change on performance and health have been documented, questionnaires to identify morning and evening-type adolescents are necessary. The aim of the present study was to validate a Spanish version of the Morningness-Eveningness Scale for Children (MESC) by means of several relevant psychological variables as external criteria. A sample of 623 urban high school students completed the MESC and self-reported measures of sleep behaviors, subjective alertness, physical performance, and mood. On the whole, results indicate a good validity of MESC. Significant differences in the self-reported ratings between morning and evening types were obtained by time-of-day. These results provide preliminary support for the Spanish version of MESC.