Glymphatic System

Every organ in the body clears its own metabolic waste through the lymphatic system, a network of vessels that drains fluid and debris into the bloodstream. The brain has no traditional lymphatic vessels, so for decades scientists assumed it relied on cerebrospinal fluid slowly diffusing waste away on its own. In 2012, a research team led by Maiken Nedergaard at the University of Rochester described a more active pathway: cerebrospinal fluid is pumped along channels that run alongside blood vessels, sweeping through brain tissue and carrying metabolic waste, including amyloid beta, out of the brain. Because the pathway borrows the brain's blood vessel network to move fluid the way lymphatic vessels move it everywhere else in the body, the researchers named it the glymphatic system.

This clearance system is not running at full capacity all the time. A 2013 study from the same lab found that during natural sleep, the space between brain cells expands by roughly 60%, which lets cerebrospinal fluid flow through brain tissue far more easily than it can while awake. That expansion is largest during deep, slow-wave sleep, the stage that dominates the first few hours of the night. More recent research suggests this flow is driven by slow, rhythmic pulses of norepinephrine, a brain chemical that also governs arousal, moving through the brain during sleep and effectively pumping fluid through the tissue.

Body position affects how well the system works. A 2015 imaging study in rodents found that fluid transport was most efficient when the body was lying on its side, compared with lying on the back or stomach. Much of the underlying research has been done in animals, and the human picture is still being filled in, but the basic pattern (that sleep quality, sleep depth, and sleep position all shape how effectively the brain clears its own waste) is now well established.