Primary Sjoegren’s syndrome (pSS) is a systemic autoimmune disease characterized by lymphocyte infiltration and gradual destruction of exocrine glands. Additionally, the inflammatory process can affect organs such as the kidneys, lungs, and heart, leading to renal decline, dyspnea, and cardiac symptoms. Sicca, particularly dry eyes and mouth, is present in 95% of patients. Moreover, 65-70% of patients report fatigue. Fatigue, described as a subjective and unpleasant sensation ranging from tiredness to exhaustion, is the most significant challenge for pSS patients. Till now, no therapeutics have been approved for the treatment of fatigue. Studies have shown that pSS patients with lower physical activity, higher activity avoidance, and greater somatic focus display a higher level of fatigue. Potentially, interventions to increase aerobic capacity and levels of physical activity may improve fatigue symptoms of pSS patients. The overarching aim of the project is to analyze whether resting heart rate as a valid population-level biomarker of cardiorespiratory fitness serves as a predictor of fatigue by continuously monitoring of heart rate in pSS patients with the use of wearables and apps in their home environment. Since fatigue is an early symptom in autoimmune diseases, especially in pSS, understanding the underlying mechanisms and the impact of high or low resting heart rate could inform the development of effective interventions to improve long-term patient outcomes and potentially delay the progression of autoimmune diseases.