The primary purpose of this study is to explore the underlying mechanisms that link sleep to Alzheimer's disease (AD), with special focus on the clearance of metabolites in the extracellular space of the brain during sleep. Subjects will wear an actigraph for 1 week to determine regular daytime activity and sleep patterns. Subjects will then undergo partial sleep deprivation followed by 4 hours of in-lab nocturnal polysomnography (NPSG). Participants will be be asked to stay awake and active all day after the partial sleep deprivation with a new actigraphy secured by a hospital band to assure participants remain awake. They will seep inside an MRI machine for 90 minutes on the following night during their usual bedtime (established by 1 week actigraphy study.) Morphologic imaging, flow imaging and diffusion kurtosis imaging (DKI) will be performed on a 3T Siemens scanner.