An exploratory study by researchers at the University of Arizona and its Cancer Center in Tucson examined the efficacy of a stress reduction intervention for women with breast cancer, and more specifically, examined the effects on their stress-related sleep complaints. Analyses of the data indicated that both MBSR and a free choice control condition produced significant improvement on daily diary sleep quality measures, though neither showed significant improvement in sleep-efficiency. Participants in the MBSR who reported greater mindfulness practice improved significantly more on the sleep quality measure most strongly associated with distress. Researchers say the diagnosis of breast cancer, the most common type of cancer among American women, elicits greater distress than any other diagnosis, regardless of prognosis. Sleep disturbance is a commonly-reported problem among cancer patients.