Date Time Increased blood flow during sleep tied to critical brain function Blood flow and neural activity greatly increase during both non-rapid eye movement sleep and rapid eye movement sleep stages, according to Penn State researchers. Image: iStock/@Hank Grebe Our brains experience significant changes in blood flow and neural activity during sleep, according to Penn State researchers. Such changes may help to clean out metabolic brain waste that builds up during the day. “We studied the sleep patterns of mice during both rapid eye movement and non-rapid eye movement sleep stages, as well as in different alertness states,” said Patrick Drew, Huck Distinguished Associate Professor of Engineering Science and Mechanics, Neurosurgery and Biomedical Engineering.