Predicting daily pain fluctuations in fibromyalgia: The role of mental distress and sleep heart-rate.

Fibromyalgia (FM) is a chronic pain condition that typically involves sharp fluctuations in pain intensity over short time periods. Both mental distress and sleep disturbances have been implicated in FM, yet their putative contributions to pain fluctuations have not been fully defined. We assessed simultaneous fluctuations in pain intensity, mental distress and sleep physiology and behavior among FM patients in natural settings. Specifically, seventy-nine adult FM patients reported their current pain and mental distress levels three times a day (morning, afternoon, evening) over two consecutive weeks via a dedicated smartphone app. During this period, sleep physiology (sleep heart rate) and behavior (sleep duration & efficiency) were continuously monitored using a wearable sensor. Mixed model analyses characterized the contribution of mental distress and sleep to daily pain fluctuations. Results revealed that current levels of pain and mental distress both predict subsequent pain levels, yet current pain does not predict subsequent mental distress. These associations were consistent across different daily segments (morning to afternoon, afternoon to evening), but not from evening to following morning. Instead, sleep heart rate emerged as predictor of next day pain, such that nights with lower vs. higher sleep heart rate were associated with pain improvement vs. worsening, respectively, in the subsequent day. These results suggest that both mental distress and sleep play a role in determining pain dynamics in FM. The contribution of sleep heart rate to pain reduction or intensification in the following day highlights the role of sleep physiology in daily pain fluctuations in FM. PERSPECTIVE: This study reveals how mental distress and sleep physiology shape daily pain fluctuations in Fibromyalgia. Using wearable sensors and real-time assessments, we identified sleep heart-rate as a key predictor of next-day pain fluctuations. These findings highlight modifiable, non-invasive targets for improving symptom dynamics and guiding personalized treatment strategies in Fibromyalgia.