Background: A substantial part of elderly persons with dementia show rest-activity rhythm disturbances. The rest-activity rhythm is important to study in people with early-onset dementia (EOD) for rest-activity rhythm disturbances are predictive of institutionalization, and caregivers of young patients suffer from high distress. Objective: The aim of this study was to study (1) whether EOD patients have more rest-activity rhythm disturbances compared with cognitively intact adults; and (2) which factors contribute to a disturbed rhythm. Methods: We included 61 patients with EOD [mean age 61.9 (4.9) y, 41 (67%) men] and 68 cognitively intact adults [mean age 61.6 (4.5) y, 28 (41%) men]. Rest-activity rhythm was assessed by actigraphy. Results: EOD patients tended to have higher intradaily variability [0.46 (0.16) and 0.39 (0.10), P=0.03]. EOD patients also lay for a longer time in bed [time in bed: 08:49 (0:51) h and 08:07 (0:47) h, P<0.001] and needed more time to fall asleep [sleep onset latency: 23 (22) min and 15 (15) min, P=0.02]. Disturbances in the restactivity rhythm were predicted by a low level of physical activity, use of antidepressants and central nervous system neurological medications, and being male. Conclusions: EOD patients showed more variability in the restactivity rhythm compared with cognitively intact adults. The main predictor for rest-activity rhythm disturbances was a low level of physical activity.