British researchers discovered that during sleep brain cells shrink. When the cells shrink the spaces between cells increases. This allows waste products to easily leave cells. The waste is carried away by the lymphatic system.
The lymphatic system consists of ducts, vessels and nodes and is part of the circulatory system. The lymphatic system collects fluid that leaks from cells and carries this fluid towards the heart.
During sleep, the lymphatic system in the brain becomes more active. This increase in activity is thought to be caused by metabolic waste leaving brain cells. Read more at http://www.bbc.com/news/health-24567412.
To learn more about the lymphatic system visit http://www.mayoclinic.org/diseases-conditions/swollen-lymph-nodes/multimedia/lymphatic-system/img-20007995