Sleep is important for brain health, having both a restorative function and playing an important role in cognitive functions, e.g., attention, memory, learning, and planning. This review finds that sleep disturbances are prevalent and associated with poorer cognitive functioning in neurodegenerative disorders such as Parkinson's disease and in people with non-neurodegenerative diseases such as cancer and mood disorders. Screening for and treating sleep disturbances are potential supplementary approaches to preventing and treating cognitive impairment.