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Given the known detrimental effects of poor sleep on an array of psychological and physical health processes, it is critical to understand the factors that protect sleep, especially during times of stress when sleep particularly suffers. Positive affect (PA) arises as a variable of interest given its known associations with health and health behaviors and its ability to buffer stress. In 2 studies, we examined which types of PA (distinguished by arousal level and trait/state measurement) were most beneficial for sleep and whether these associations varied depending on the stress context. In Study 1, college students (N = 99) reported on their PA and sleep during the week of a major exam. In Study 2, 2 weeks of daily PA and sleep data were collected during a period with no examinations in a similar sample of students (N = 83). Results indicated that high trait vigor was tied to better sleep efficiency and quality, especially during high stress. Trait calm was generally unhelpful to sleep, and was related negatively to sleep duration. State calm, on the other hand, interacted with stress in Study 2 to predict more efficient day-to-day sleep on days with higher average stress. These findings illustrate the importance of considering arousal level, affect duration, and the stress context in studies of PA and health.