This study explores how employees’ flow experience at work emerges, is sustained, and continuously grows over time. Based on the job demand-resource model, we propose the intraday upward spiral of flow: Challenging demands and job resources activate employees’ flow experience, further encouraging them to seek more challenges and resources. Furthermore, drawing on the perseverative cognition theory and spill-crossover model, we propose the inter-day upward spiral of flow: The antecedents (or consequences) of flow can overflow from work to the family domain and result in employees’ positive rumination, thus promoting the next-day flow experience. Our diary study generated 1,208 data points from 142 employees over 10 working days. We found that in the morning, challenging demands and job resources positively affected the participants’ flow, further encouraging them to pursue more challenging demands and job resources in the afternoon and thus enter this state again. Moreover, the afternoon’s challenging demands and job resources promoted the respondents’ problem-solving pondering at night, which further increased their next-morning challenging demands, job resources, and, thus, their flow. Through this study, we expand the emerging literature on positive organizational behavior and provide information for practitioners on how to build and sustain employees’ peak states.