Does the brain flush out toxins while you sleep?

We spend one-quarter to one-third of our lives sleeping, but while we rest, the brain remains hard at work, performing regular maintenance. Sleep is known to be important for learning and memory creation, and you may have heard that, during sleep, the brain rids itself of waste built up while we are awake. 

But how does the brain flush these toxins out of its system, and why does this happen only during sleep?

The brain's waste-disposal apparatus is known as the glymphatic system, which includes a network of tunnels that surround blood vessels in the brain. This system is an analog of the waste clearance system found in the rest of the body, called the lymphatic system.

The tunnels in the glymphatic network contain a clear, watery substance called cerebrospinal fluid (CSF), which contains nutrients and physically cushions the brain. CSF from these tunnels into the spaces of the brain, where it mixes with another fluid found between active brain cells.

Related: How does the brain store memories?

Here, the CSF picks up metabolic waste from those cells; this includes amyloid-beta proteins, which build up in the brain in Alzheimer's disease. The "dirty" fluid then gets flushed out of the space and exits the brain through lymphatic vessels that funnel the CSF into the lymphatic system, where it's cleared away.

Aside from clearing away this waste, the glymphatic system helps transport fats, sugars and chemical messengers in the nervous system and may play a role in distributing drugs within the brain.