Introduction: In this paper we investigate how students' class grades are affected by individual differences in circadian rhythm, class time-of-day and class difficulty. Method: Using a sample of university students, we assessed morningness and eveningness personality type, and then obtained students recalled classes as well as their university-recorded classes and grades. Results and Conclusions: The results of our analysis revealed that, for very difficult classes, "optimal-time-of-day" played a significant role in determining students' grades. Our results also suggest that eveningness-type individuals appear to be most affected by standard class times. Analysis of our findings also revealed optimal-time-of-day effects for recall of the previous semester's classes. Aside from obvious implications for students, we mention implications also for teachers and administrators. (Contains 2 tables.)