"The first study in this survey of the impact of attendance on student classroom success is
by Robert M. Schmidt ("Who Maximizes What? A Study in Student Time Allocation ", AMERICAN ECONOMIC REVIEW, May, 1983, pp. 23-28). In this study, the author measured the impact of time commitments by students to various course activities on the students' performance in the given class. The results were revealing.
By far, the most valuable and important time commitment in a course was the time actually spent in the classroom. That time was the most important determinant of student success and each unit of time in the class itself provided, among all the class related activities, the greatest improvement in student performance.
The next most important time spent on a class was any time spent in discussion sections that accompanied the lectures. Third in importance was any time spent studying outside of class preparing for the class session itself. Perhaps most surprising was the result that the least significant time commitment in improving student performance in a particular class was the time spent studying for the final exam.
Thus the study concludes that the most productive time in any course is the time actually spent in the classroom. That time has the greatest positive impact on overall student performance. The hour or two spent in class each day (for a particular course) does the most to improve the student's grade."
http://www.mnsu.edu/cetl/teachingresources/articles/classattendance.html
In undergrad if I missed class, I just ended up sleeping. I always did considerably better in the classes I attended. In my opinion it really depends on how you use your time. If you skip lecture from 8-12 to study your notes, you will be good. But if you sleep in...thats 4 hours of study time lost. Unless of course you plan on going to sleep 4 hours later.