Objective: Previous findings indicate that patients with chronic fatigue syndrome (CFS) report significant day-to-day fluctuations in subjective energy and sleep. Herein, we examined whether daily variation in the satisfaction and frustration of the basic psychological needs for autonomy, competence, and relatedness would contribute to daily variation in subjective energy and quality and quantity of sleep. In addition, we examined whether daily variation in sleep would contribute to daily need-based experiences through (i.e., mediated by) daily fluctuations in subjective energy. Method: CFS patients (N = 120; 92% female; M-age = 42.10 years, SD = 10.46) completed a diary for 14 days which assessed their need-based experiences and subjective energy every evening and sleep every morning. Results: Results indicated that subjective energy, sleep, and need experiences fluctuated significantly from day to day. Daily need satisfaction related to less daily fatigue and more daily vitality, while the opposite pattern was observed for daily need frustration. Daily need frustration was also uniquely related to poorer daily sleep quality. Lastly, better daily sleep quality was also uniquely related to more daily need satisfaction and less daily need frustration via (i.e., mediated by) daily variation in subjective energy. These reciprocal within-day associations remained significant after controlling for the previous day's level of each outcome, with the exception of the relation between need frustration and sleep quality. Conclusion: The present findings underscore the reciprocal day-to-day association between need-based experiences and subjective energy in CFS.