Mun CJ, et al. - Given that in musculoskeletal pain disorder fibromyalgia, nonrestorative sleep is a key diagnostic feature and is observed to have a robust association with poor physical functioning, including activity interference, researchers sought to assess the mechanisms through which nonrestorative sleep evokes activity interference among individuals with fibromyalgia at the within-person level. They gathered data from a 21-day electronic daily diary in 220 individuals with fibromyalgia to test the following 3-path mediation model: previous night nonrestorative sleep → morning pain catastrophizing → afternoon pain severity → end-of-day activity interference. The 3-path mediation model was supported by the outcomes of multilevel structural equation modeling.Sequential mediation of daily nonrestorative sleep and activity interference was observed. Higher morning pain catastrophizing was evident in correlation with greater nonrestorative sleep. They noted significant predictive value of morning pain catastrophizing for afternoon pain severity. Greater end-of-day activity interference with higher afternoon pain severity was evident. Further, nonrestorative sleep and catastrophizing uniquely prognosticate activity interference. These findings support attenuating nonrestorative sleep and pain catastrophizing to increase the daily physical functioning of this population.