STUDY OBJECTIVES: Brain regions involved in insomnia and chronic pain are overlapping and diffuse. The interactive role of physiological arousal in associations between insomnia symptoms and neural regions is unknown. This preliminary study examined whether arousal interacted with sleep in associations with gray matter (GM) volume of frontal [dorsolateral prefrontal cortex (DLPFC), anterior cingulate cortex (ACC)] and temporal [right/left (R/L) hippocampus] regions in adults with chronic widespread pain and insomnia complaints (CWPI). METHODS: Forty-seven adults with CWPI (M(age)=46.00, SD=13.88, 89% women) completed fourteen daily diaries measuring sleep onset latency (SOL), wake time after sleep onset (WASO) and total sleep time (TST), as well as Holter monitor assessments of heart rate variability (HRV, measuring physiological arousal), and magnetic resonance imaging. Multiple regressions examined whether average SOL, WASO or TST were independently or interactively (with arousal/HRV) associated with DLPFC, ACC, and left/right hippocampus GM volumes. RESULTS: Shorter TST was associated with lower R hippocampus volume. TST also interacted with arousal in its association with R hippocampal volume, Specifically, shorter TST was associated with lower volume at highest and average arousal levels. SOL interacted with arousal in its association with ACC volume, such that among individuals with lowest arousal, longer SOL was associated with lower volume. CONCLUSIONS: Preliminary findings highlight the interactive roles of physiological arousal and insomnia symptoms in associations with neural structure in CWPI. Individuals with highest physiological arousal may be particularly vulnerable to the impact of shorter TST on hippocampal volume loss. Reducing SOL may only impact ACC volume in those with lower physiological arousal.