The literature describes research comparing scholastic success in the morning and afternoon in elementary and high schools. The present study examines the relationship between time of day and scholastic performance in middle school. The progress of 850 seventh and eighth grade students in academic subjects taught at different hours of the day was studied. Mean achievement, as expressed in final grades, rose moderately from morning lessons to those conducted at later hours. Significant but temporary declines in achievement were observed immediately after the 10:00 recess and again during the 13:00 lesson. Variance within classes increased sharply during the day. Previous studies attributed such changes to biological rhythms. This paper indicates that many diurnal changes in scholastic performance may be clarified by integration of concepts of biological rhythms with current theories with respect to attention level. Implications of these findings for planning daily school schedules are discussed.