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Sleep flushes waste from the brain
The brain has no valves like the lymph system in the body. In sleep, astrocytes, a type of glial cell, create a bit of extra space between neural tissue, so waste products flow up and out toward the surface of the brain and then drain away. The name blends glia and lymph. Some of it comes out through the nose.
When it falls short, lymph and other fluids and waste products accumulate, which is where bags under the eyes come from after a night of bad sleep. Researchers once thought a slightly inverted position helped, and now think slightly elevated is better.
A recent paper in the Proceedings of the National Academy of Sciences found that a short meditation increases glymphatic clearance in the daytime. A drive for that clearance may also be one reason people sleep a lot more when they're sick.
How was the glymphatic system discovered?
In 2012, Maiken Nedergaard's lab at the University of Rochester used two-photon microscopy in living mice to watch fluid move through the brain. Cerebrospinal fluid flowed along channels around blood vessels and mixed with fluid between cells. A 2013 follow-up paper in Science found the space between brain cells grew by about 60 percent in sleeping mice. Nedergaard's team named the system for the glial cells that run it.
What waste does sleep clear out?
The flushed waste includes amyloid-beta, the protein that forms plaques in Alzheimer's disease. The 2013 mouse study found it cleared faster during sleep than during waking. In humans, an NIH study led by Ehsan Shokri-Kojori, published in PNAS in 2018, found that a single night without sleep raised amyloid levels in brain scans. The protein is a normal by-product of neural activity.
Does the brain have lymph vessels?
For decades, textbooks said no vessels carried lymph from the brain. In 2015, Jonathan Kipnis's lab at the University of Virginia reported lymphatic vessels in the membranes that wrap the brain in mice. The vessels carry fluid and immune cells to lymph nodes in the neck. The finding forced researchers to redraw the picture of how the brain connects to the immune system.
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