OBJECTIVES To explore factors related to sleep disturbance in patients with rheumatoid arthritis (RA). METHOD Cross-sectional data from 986 patients in the Oslo RA Register (ORAR) collected in 2009 were included. Sleep problems were assessed by four measures: the Medical Outcomes Study (MOS) sleep disturbance scale, and the sleep components of the Rheumatoid Arthritis Impact of Disease (RAID) score, the Multi-Dimensional Health Assessment Questionnaire (MDHAQ), and the 15-dimensional quality of life questionnaire (15D). Patient-reported outcomes (PROs) were recorded using standard questionnaires for physical and mental function [the HAQ and the MOS 36-item Short-Form Health Survey (SF-36), disease activity (the RA Disease Activity Index, RADAI), utility (SF-6D), and visual analogue scales (VAS) for pain, fatigue, and disease activity]. Demographics including comorbidity were collected. Information on use of medication for RA and sleep disturbance was obtained using checklists. Multivariate analyses were used to identify factors independently associated with sleep problems by four different measures. RESULTS The mean (standard deviation, SD) age of the patients was 59.4 (12.5) years, 76.9% were females, and the mean (SD) disease duration was 13.7 (10.7) years. The correlation between the various sleep measures was high (r2 = 0.71-0.78). Sleep disturbance was moderately correlated to pain (r2 = 0.41-0.61), fatigue (r2 = 0.44-0.58), physical function (r2 = 0.33-0.48), RADAI (r2 = 0.42-0.55), and utility (r2 = 0.49-0.61). RAID sleep demonstrated the highest correlation with other PROs. RADAI, fatigue, the mental component score of SF-36, physical function, body mass index (BMI), and use of Z-drugs/benzodiazepines were independently associated with two or more measures of sleep problems (all p < 0.001). CONCLUSIONS Sleep disturbance measured by four different measures was independently related to other PROs including fatigue, pain, and disease activity in RA patients.