STUDY OBJECTIVES: Primary dysmenorrhea is a common gynecological disorder that disrupts daytime functioning and nighttime sleep quality. We determined the effectiveness of diclofenac potassium, compared to placebo, in alleviating nighttime pain and restoring sleep architecture in women with primary dysmenorrhea. DESIGN: Randomized, double-blind, crossover study. SETTING: Sleep laboratory. PARTICIPANTS: Ten healthy women (21 +/- 1 years) with a history of primary dysmenorrhea. INTERVENTIONS: Placebo or diclofenac potassium (150 mg per day) for menstrual pain. MEASUREMENTS AND RESULTS: We assessed objective measures of sleep (polysomnography) and subjective measures of sleep quality, mood, and intensity of menstrual pain. Compared to a pain-free phase of the menstrual cycle (mid-follicular), women receiving placebo for their menstrual pain had a poorer mood (P < 0.01), decreased sleep efficiency (P < 0.05), less REM sleep (P < 0.05), more stage 1 sleep (P < 0.01), and more sleep stage changes per hour of sleep during the night. Administration of diclofenac potassium compared to placebo not only attenuated the women's menstrual pain (P < 0.05), but also increased sleep efficiency (P < 0.05) and percentage of REM sleep (P < 0.01), decreased percentage of stage 1 sleep (P < 0.05) and number of sleep stage changes per hour of sleep (P < 0.05), and improved subjective ratings of sleep quality and morning vigilance (P < 0.05). CONCLUSION: Diclofenac potassium effectively attenuates nighttime dysmenorrheic pain and restores subjective and objective measures of sleep quality to values recorded in a pain-free phase of the menstrual cycle.