Poor sleep is associated with several negative consequences, including poor health, depression, anxiety, and memory deficits, among others. Although the link from sleep to health and wellbeing is well-established, fewer studies have examined the reverse relationship. The current study examined the role of one particular challenge to well-being, stressful uncertainty, in the association between well-being and sleep quantity and quality. Female patients (n = 120 for the purpose of analyses) awaiting the results of a breast biopsy participated in an initial interview at their biopsy appointment and then completed daily surveys at home each day until they received their results. Patients who reported poorer well-being on various measures also reported poorer and less sleep on average during the wait for biopsy results, even after controlling for individual differences and well-being at the biopsy appointment. However, when patients experienced positive emotions on a given day, they tended to sleep better that night. Our findings suggest that stressful uncertainty about one's health may have detrimental effects on sleep, but positive emotions may improve sleep during stressful waiting periods.