Shorter duration of sleep has been associated with risk of a number of medical conditions, including breast cancer. However, no prior study has investigated the relationship of average sleep duration before diagnosis and cancer aggressiveness. OncotypeDX is a widely utilized test to guide treatment in early stage hormone receptor positive breast cancer by predicting likelihood of recurrence. We reviewed medical records from ER+ early stage breast cancer patients participating in a case–control study for availability of OncotypeDX scores. All patients in the parent study were recruited at diagnosis and asked about average sleep duration in the 2 years before diagnosis. We analyzed data from 101 breast cancer patients with available OncotypeDX recurrence scores to test the hypothesis that shorter sleep is associated with greater likelihood of recurrence. We found that OncotypeDX recurrence scores were strongly correlated with average hours of sleep per night before breast cancer diagnosis, with fewer hours of sleep associated with a higher (worse) recurrence score (R = −0.30, p = 0.0031). This correlation was limited to post-menopausal breast cancer patients only (R = −0.41, p = 0.0011, for postmenopausal patients; R = −0.05, p = 0.80 for pre-menopausal patients). This association remains statistically significant after adjustment for age, physical activity, smoking status, and body mass index in the entire study sample (p = 0.0058) as well as in postmenopausal patients (p = 0.0021). This is the first study to suggest that women who routinely sleep fewer hours may develop more aggressive breast cancers compared with women who sleep longer hours.