Different models have been developed to describe human experiences. One of these models is the core affect model, which states that the core of emotional experience is a simple state of feeling good or bad, energized or drowsy. Another model is the experience fluctuation model (EFM), which has been developed to map how emotions and other experiences fluctuate along with the relationship between challenges and skills. In this study, we first mathematically refined the EFM. Second, we tested if fluctuations in core affect could be modeled with a continuous wave-like sine function (Study 1). Third, we applied the sine function to our own experience-sampling data, which we collected via mobile phones from 55 university students, and located each challenge–skill relationship in a two-dimensional core affect space (Study 2). The results were consistent in both studies and showed that fluctuations in core affect dimensions, measured as a function of the relationship between challenges and skills, can be modeled with the sine function. The results also indicated that there is a systematic link between challenge–skill relationships and core affect: High challenge–high skill situations were connected to a very active and pleasant core affect, whereas high challenge–low skill situations were connected to a quite active but unpleasant core affect, low challenge–low skill situations to a very passive and unpleasant core affect, and low challenge–high skill situations to a quite passive and pleasant core affect. Compared to previous studies, this study presents a more fine-grained and comprehensive level of information on the relationship between core affect and challenges and skills.