The current study examined sleep disturbance (ie, sleep duration, sleep quality) as a correlate of stress reactivity and pain reactivity. An ecological momentary assessment design was used to evaluate the psychosocial functioning of men and women with fibromyalgia or rheumatoid arthritis (N= 49). Participants recorded numeric ratings of pain, the occurrence of a stressful event, as well as positive and negative affect 7 times throughout the day for 2 consecutive days. In addition, participants reported on their sleep duration and sleep quality each morning. Sleep disruption was not found to be an independent predictor of affect. However, sleep was found to buffer the relationship between stress and negative affect and the relationship between pain and both positive and negative affect. These results are consistent with a model in which good-quality sleep acts as a biobehavioral resource that minimizes allostatic load.(PsycInfo Database Record (c) 2021 APA, all rights reserved)