The connection between stress and sleep is well established in cross-sectional questionnaire studies and in a few prospective studies. Here, the intention was to study the link between stress and sleep on a day-to-day basis across 42days. Fifty participants kept a sleep/wake diary across 42days and responded to daily questions on sleep and stress. The results were analyzed with a mixed model approach using stress during the prior day to predict morning ratings of sleep quality. The results showed that bedtime stress and worries were the main predictors of sleep quality, but that, also, late awakening, short prior sleep, high quality of prior sleep, and good health the prior day predicted higher sleep quality. Stress during the day predicts subsequent sleep quality on a day-to-day basis across 42 days. The observed range of variation in stress/worries was modest, which is why it is suggested that the present data underestimates the impact of stress on subsequent sleep quality.