Sleep may jumpstart the brain’s power washing system

During sleep, waves of fresh fluid wash over the brain every 20 seconds, a new study finds. These slow, rhythmic blasts may help explain why sleep is so important for brain health.

Studies in mice have shown that the waves can wash away proteins that build up between brain cells. The new results strengthen the idea that a similar power wash happens in sleeping people.

Researchers scanned the brains of 13 healthy young people as they slept. This magnetic resonance imaging, or MRI, tracked fluid as it flowed in and out of the brain. That fluid surrounds and protects the brain and spinal cord.

Fresh fluid flowed into sleeping brains like ocean tides, the imaging showed. The pattern was obvious — and big, says Laura Lewis. She is a neuroscientist and engineer in Massachusetts at Boston University.

Small, gentle waves of fluid flow through the brains of people who are awake. These waves are linked to breathing patterns. The sleep waves were more like tsunamis, Lewis found. “The waves we saw during sleep were much, much larger, and higher velocity,” she explains.

Before those tsunami-like waves wash in, other types of waves sweep the brain, Lewis’ team found. First, groups of nerve cells send a slow wave. This electrical activity signals a certain type of sleep known as non-REM. Then, levels of oxygen in the brain’s blood fall. This is a sign that blood is leaving the brain. Finally, the fluid rolls into the brain. This wave may take the place of the exiting blood. Lewis and her team shared their new discovery November 1 in Science.

The various waves are linked, Lewis says, but it’s not yet clear how. Her team plans on testing whether one event causes the others.

These powerful waves of fluid may clear harmful wastes from the sleeping brain, says Maiken Nedergaard. She is a neuroscientist at the University of Rochester Medical Center in New York. Her team found that waves of fluid coming into the brains of mice can carry away bits of a protein called amyloid-beta. This sticky protein may muck up the brains of people with Alzheimer’s disease. That disease impairs the ability to think, learn and remember.

As mice sleep, more fluid enters their brains. And more of the protein gets cleared away, Nedergaard’s studies show. Finding a similar influx of fluid in sleeping humans “is really a significant move,” she says.

Lewis says that studying these fluid waves in people with Alzheimer’s disease might reveal new aspects of the disorder. Slow waves during sleep tend to decline with age. And the decline is severe in people with Alzheimer’s disease. Fewer of the electrical slow waves could mean fewer fluid waves, Lewis says. That could allow more toxic proteins to stick around.