Abstract Introduction Suboptimal sleep has been consistently associated with greater next-day pain intensity among women with fibromyalgia. In contrast, associations between physical activity and same-day pain in this population are contradictory. Given this inconsistency, we aimed to determine whether the daily physical activity-pain association is modified by sleep parameters. Methods This micro-longitudinal study used data collected using wrist-worn triaxial accelerometers over seven consecutive days by 44 adult women with fibromyalgia. Derived variables included sleep duration, sleep onset latency (SOL), wake after sleep onset (WASO), sleep efficiency (SE) and maximum daily physical activity count. Participants also completed digital diaries of refreshed sleep upon awakening (scale 0-100), and pain intensity (5x/day, scale 0-100). Multilevel linear regression models with interaction terms were used to investigate moderating effects of sleep on the next-day person-centered maximum physical activity-average pain intensity association. Results The sample mean age was 44 (SD 14). A total of 304 days of data were available for analysis. Mean sleep duration was 471min(SD 66); mean SOL 10min(SD 10); mean WASO 31min(SD 17), and mean SE 90%(SD 5). Sleep duration moderated the next-day maximum physical activity-pain association (Wald statistic p=0.01). After nights of both shorter (<7 hours) and longer sleep (>8 hours), higher levels of next-day maximum physical activity (compared to the participant’s overall study average) were associated with days of greater average pain. In contrast, after nights of 7-8 hours of sleep, higher levels of next-day maximum physical activity were associated with days of lower average pain. Conclusion An association between higher maximum physical activity and lower levels of pain was only observed after nights of 7-8 hours sleep. Engaging in physical activity is recommended for fibromyalgia-related pain management. Optimizing sleep duration may be useful in minimizing physical activity-related pain in this clinical population. Support DW is supported by a Foundation Fellowship Versus Arthritis. The study that provided data was supported by the National Institute of Arthritis and Musculoskeletal and Skin Diseases of the National Institutes of Health (award number K01AR064275;PI:ALK). The Michigan Institute for Clinical & Health Research provided subject recruitment support through the UMHealthResearch.org website (MICHR:NIH award number UL1TR002240).