Positive affect and pain: mediators of the within-day relation linking sleep quality to activity interference in fibromyalgia.

Fibromyalgia (FM) is a chronic pain condition often resulting in functional impairments. Nonrestorative sleep is a prominent symptom of FM that is related to disability, but the day-to-day mechanisms relating the prior night's sleep quality to next-day reports of disability have not been examined. This study examined the within-day relations among early-morning reports of sleep quality last night, late-morning reports of pain and positive and negative affect, and end-of-day reports of activity interference. Specifically, we tested whether pain, positive affect, and negative affect mediated the association between sleep quality and subsequent activity interference. Data were drawn from electronic diary reports collected from 220 patients with FM for 21 consecutive days. The direct and mediated effects at the within-person level were estimated with multilevel structural equation modeling. Results showed that pain and positive affect mediated the relation between sleep quality and activity interference. Early-morning reports of poor sleep quality last night predicted elevated levels of pain and lower levels of positive affect at late-morning, which, in turn, predicted elevated end-of-day activity interference. Of note, positive affect was a stronger mediator than pain and negative affect was not a significant mediator. In summary, the findings identify 2 parallel mechanisms, pain and positive affect, through which the prior night's sleep quality predicts disability the next day in patients with FM. Furthermore, results highlight the potential utility of boosting positive affect after a poor night's sleep as one means of preserving daily function in FM.