Abstract. Recent advances in neuro-physiological measurements resulted in reliable and objective measures of Cognitive Load (CL), e.g., using pupillary responses. However, continuous measurement of CL in software design activities, e.g., conceptual modeling, has received little attention. In this paper, we present the progress of our work intended to close this gap by continuously measuring cognitive load during design activities. This work aims at advancing our understanding of WHEN and WHY designers face challenges. For this, we attempt to explore and explain the occurrence of CL using fine-granular units of analysis (e.g., type of subtasks, evolution of design artifact's quality, and manner of technology use). We expect implications for the future development of intelligent software systems, which are aware WHEN a particular designer experiences challenges, but also WHY challenges occur.