How Sleep Clears the Brain

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The scientists also reported that the glymphatic system can help remove a toxic protein called beta-amyloid from brain tissue. Beta-amyloid is renowned for accumulating in the brains of patients with Alzheimer’s disease. Other research has shown that brain levels of beta-amyloid decrease during sleep. In their new study, the team tested the idea that sleep might affect beta-amyloid clearance by regulating the glymphatic system. The work was funded by NIH’s National Institute of Neurological Disorders and Stroke (NINDS).

The researchers first injected dye into the cerebrospinal fluid of mice and monitored electrical brain activity as they tracked the dye flow through the animals’ brains. As reported in the October 18, 2013, edition of Science, the dye barely flowed when the mice were awake. In contrast, when the mice were unconscious — asleep or anesthetized — it flowed rapidly.Changes in the way fluid moves through the brain between conscious and unconscious states may reflect differences in the space available for movement. To test the idea, the team used a method that measures the volume of the space outside brain cells. They found that this “extracellular” volume increased by 60% in the brain’s cortex when the mice were asleep or anesthetized.

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