To examine sleep quality and self-reported causes of sleep disturbance among patients with breast cancer at diagnosis and one year later. 486 of 606 patients with histologically confirmed breast cancer completed a Pittsburgh Quality Sleep Index (PSQI) survey at the time of diagnosis and again one year later. In this secondary data analysis, descriptive statistics were computed for seven PSQI components and its global score. Wilcoxon signed-rank tests and McNemar's tests were used. Self-reported reasons for sleep disturbances were summarized. PSQI scores significantly increased from baseline (mean = 6.75) to one-year follow-up (mean= 7.12), indicating worsened sleep. Sleep disturbance and onset latency scores increased, whereas sleep efficiency decreased. The two most frequently reported reasons for sleep disturbance were waking up late in the night or early in the morning (more than 50%) and needing to use the bathroom (49%). Feeling too hot and experiencing pain three or more times per week were reported by participants at baseline and one year later. Results can aid in monitoring patient response to treatment methods and formulating benchmarks to manage sleep problems.