The current paper examines the effectiveness of offering a small amount of extra credit as an incentive to encourage proper work distribution and reduce procrastination and cramming among college students completing introductory physics homework assignments in the form of online learning modules. Students' distribution of work over time is systematically measured by clustering clickstream log events into study sessions according to a cutoff determined empirically using mixture model analysis. Significantly more study sessions are initiated well before the assignment due date when extra credit is offered compared to data from a previous semester. Using two proxy variables designed to capture the distribution and duration of work, we found that in addition to starting the assignments earlier, students also spent a longer time on the assignments. Finally, the benefit of extra credit in encouraging work distribution is not limited to high-performing students, as shown by a reduction in score gap between early and late starters on a midterm exam administered prior to the release of the homework assignment.