Background: Medical residency can be a time of increased psychological stress and sleep disturbance. We examine the prospective associations between self-reported sleep quality and resident wellness across a single training year. Methods: Sixty-nine (N = 69) resident physicians completed the Brief Resident Wellness Profile (M = 17.66, standard deviation [SD] = 3.45, range: 0 = 17) and the Pittsburgh Sleep Quality Index (M = 6.22, SD = 2.86, range: 12 = 25) at multiple occasions in a single training year. We examined the 1-month lagged effect of sleep disturbances on residents' self-reported wellness. Results: Accounting for residents' overall level of sleep disturbance across the entire study period, both the concurrent (within-person) within-occasion effect of sleep disturbance (B = -0.20, standard error [SE] = 0.06, p = 0.003, 95% confidence interval [CI]: -0.33, -0.07) and the lagged within-person effect of resident sleep disturbance (B = -0.15, SE = 0.07, p = 0.037, 95% CI: -0.29, -0.009) were significant predictors of decreased resident wellness. Increases in sleep disturbances are a leading indicator of resident wellness, predicting decreased well-being 1 month later. Conclusions: Sleep quality exerts a significant effect on self-reported resident wellness. Periodic evaluation of sleep quality may alert program leadership and the residents themselves to impending decreases in psychological well-being.