The main objective of this study was to examine daily associations between sleep and mood within a midlife adult population as this period of life has received very little attention. Much of the existing literature covers characteristics of sleep in both young adults and during old age, or tests associations between sleep and psychological disorders and physical illness. Although sleep quality begins to decline in early adulthood, sleep characteristics become more stable by old age, and the greatest changes are observed when comparing sleep between young and old adults. Little is understood about sleep in midlife adulthood, particularly as it affects daily functioning in a community-based population. As middle-aged adults are busy and involved in multiple roles, many in this age group fail to obtain optimal sleep. A poor night's sleep can disrupt one's ability to manage their daily responsibilities efficiently by inhibiting optimal regulation of daily mood. In addition, increases in daily stress can exacerbate the effect of poor sleep quality on daily mood, and in turn may affect next day's mood. Using a middle-aged community sample, this study utilizes daily diary data including self-reported sleep, mood, and stress measures that were concurrently collected over 56 consecutive days. A global measure of sleep quality was also included in the measures collected. Multilevel modeling was used to assess the sleep-mood relationship at both the between-person and within-person levels with findings supporting the hypothesis that sleep plays a role in daily emotion regulation. Neither variation from one's average total night's sleep time nor the average amount of nightly sleep one received across the 56 days was found to significantly predict daily negative mood. Positive mood, on the other hand, was predicted by a quadratic trend in nightly sleep time variation, in which greater sleep time was associated with better daily positive mood. In separate analyses, however, variability in one's sleep time predicting negative and positive mood was predicated on an individual's level of global sleep quality, in which poorer global sleep quality was related to increased daily negative mood, and decreased daily positive mood. When considering the relationship between concurrently measured daily stress and daily mood, the quality of nightly sleep significantly predicted the degree to which perceptions of daily stress influenced that same day's negative mood regulation, although the effect was small. Both increased stress and poor sleep quality significantly reduced daily positive mood, but no interaction between stress and sleep was found in this relationship. A third analysis found that an intervening night's sleep quality mediated the relationship between one day's perceptions of stress and the next day's positive mood. The relationship between stress and the next day's negative mood, however, was not mediated by the quality of an intervening night's sleep, but may play role in suppressing the true relationship between a day's stress and an intervening night's sleep.. The cumulative results of this study suggest that, not only do individuals who have the best global sleep quality have the best daily mood, but the further an individual deviates from his or her normal sleep quality predicts variation in daily negative and positive mood (emotional reactivity). Interestingly, both day-to-day sleep quality and nightly amount of sleep were related to positive and negative mood in different ways.