Researchers have found that blood flow in the brain capillaries, which is important for oxygen/nutrient delivery and waste removal, was increased during rapid eye movement sleep in mice. Adenosine A2a receptors might be at least partially responsible for this increased blood flow. These findings bring new hope for understanding the function of sleep and developing treatments for neurodegenerative diseases that involve the buildup of waste products in the brain, such as Alzheimer's disease.