AbstractINTRODUCTIONSleep and rest–activity rhythm alterations are common in neurodegenerative diseases. However, their characterization in patients with behavioral variant frontotemporal dementia (bvFTD) has proven elusive. We investigated rest–activity rhythm alterations, sleep disturbances, and their neural correlates in bvFTD.METHODSTwenty‐seven bvFTD patients and 25 healthy controls completed sleep questionnaires and underwent 7 days of actigraphy while concurrently maintaining a sleep diary. Cortical complexity and thickness were calculated from T1‐weighted magnetic resonance (MR) images.RESULTSCompared to controls, bvFTD patients showed longer time in bed (95% confidence interval [CI]: 79.31, 321.83) and total sleep time (95% CI: 24.38, 321.88), lower sleep efficiency (95% CI: −12.58, −95.54), and rest–activity rhythm alterations in the morning and early afternoon. Increased sleep duration was associated with reduced cortical thickness in frontal regions.DISCUSSIONPatients with bvFTD showed longer sleep duration, lower sleep quality, and rest–activity rhythm alterations. Actigraphy could serve as a cost‐effective and accessible tool for ecologically monitoring changes in sleep duration in bvFTD patients.Highlights We assessed sleep and circadian rhythms in behavioral variant frontotemporal dementia (bvFTD) using actigraphy. Patients with bvFTD show increased sleep duration and reduced sleep quality. Patients with bvFTD show rest–activity alterations in the morning and early afternoon. Sleep duration is associated with reduced cortical thickness in frontal regions. These alterations may represent an early sign of neurodegeneration.