This study examined the effects of exam length on student performance and cognitive fatigue in an undergraduate biology classroom. Exams tested higher order thinking skills. To test our hypothesis, we administered standard- and extended-length high-level exams to two populations of non-majors biology students. We gathered exam performance data between conditions as well as performance on the first and second half of exams within conditions. We showed that lengthier exams led to better performance on assessment items shared between conditions, possibly lending support to the spreading activation theory. It also led to greater performance on the final exam, lending support to the testing effect in creative problem solving. Lengthier exams did not result in lower performance due to fatiguing conditions, although students perceived subjective fatigue. Implications of these findings are discussed with respect to assessment practices.