Despite its value in higher education, academic rigor is a challenging construct to define for instructor and students alike. How do students perceive academic rigor in their biology course work? Using qualitative surveys, we asked students to identify "easy" or "hard" courses and define which aspects of these learning experiences contributed to their perceptions of academic rigor. The 100-level students defined hard courses primarily in affective terms, responding to stressors such as fast pacing, high workload, unclear relevance to their life or careers, and low faculty support. In contrast, 300-level students identified cognitive complexity as a contributor to course rigor, but course design elements--alignment between instruction and assessments, faculty support, active pedagogy--contributed to the ease of the learning process. Overwhelmingly, all students identified high faculty support, learner-centered course design, adequate prior knowledge, and active, well-scaffolded pedagogy as significant contributors to a course feeling easy. Active-learning courses in this study were identified as both easy and hard for the very reasons they are effective: they simultaneously challenge and support student learning. Implications for the design and instruction of rigorous active-learning college biology experiences are discussed.