Adolescents tend to experience alterations in their sleep cycles inducing them to go to sleep and wake up at later times than they typically would at younger age (Delayed Sleep Phase, DSP). We asked a relatively homogeneous sample (N = 22) of female high-school students to maintain an exhaustive sleep log during two weeks. Within this period, we randomly assigned each participant to a morning or afternoon testing condition (morning vs. afternoon group), in which we measured performance data along with ERPs during a standard Stroop task. The sleep log data confirmed a two-hour DSP in both groups. Participants in the afternoon group performed significantly better (less Stroop interference) than those in the morning group and showed a differential ERP pattern to incongruent and congruent stimuli, not shown by the morning group. We conclude that DSP can be associated with an attentional impairment in the morning.