Sleep and circadian rhythms play fundamental roles in maintaining brain health throughout the lifespan. Growing evidence indicates that disturbances in sleep architecture, circadian timing, and physiological regulation are associated with cognitive decline, increased dementia risk, and progression of neurodegenerative disease. At the same time, advances in wearable sensors, sleep EEG technologies, and computational methods now enable continuous assessment of brain and physiological function in both laboratory and real-world settings, providing new opportunities to identify early markers of brain aging and disease vulnerability. A major focus of our work has been to investigate how sleep, circadian rhythms, and physiological dynamics interact to influence cognitive aging and neurodegeneration. Our studies suggest that sleep EEG can provide a unique window into large-scale brain network organization, while measures of circadian regulation, rest–activity rhythms, autonomic function, and physiological complexity offer complementary information about brain health that is not captured by conventional clinical assessments. Increasing evidence further indicates that sleep and circadian dysfunction may contribute directly to neurodegenerative processes through mechanisms involving neural network disruption, impaired glymphatic clearance, and altered protein homeostasis. In this presentation, we will discuss emerging evidence linking sleep and circadian physiology to brain aging and dementia, highlight novel physiological and digital biomarkers derived from sleep EEG and continuous monitoring technologies, and explore how multimodal assessments of physiological dynamics may advance the prediction, monitoring, and prevention of neurodegenerative disease.