Your brain works hard every day, so it makes waste. How does it clean itself?

In 2013, scientists studied mice. They found that during sleep, the spaces between brain cells became about 60 percent wider. A clear liquid flowed in and washed waste away.

But in 2024, another team found the opposite result. They said cleaning was slower during sleep. Scientists are still arguing. But sleep is still important for your brain.

The brain uses a lot of energy, so it produces waste, but it has almost no lymph vessels to remove it. In 2012, a research team in the US reported that in mice, a clear liquid called cerebrospinal fluid flows into the brain along spaces around blood vessels and washes waste away. They named this the glymphatic system.

In 2013, the same team found that during sleep, the spaces between brain cells became about 60 percent wider, and waste linked to Alzheimer's disease was removed faster. This made the idea that sleep cleans the brain very popular. However, in 2024, a team in London measured clearance in a different way and found that it was actually lower during sleep. The debate has not been settled.

Although the brain consumes a large share of the body's energy, it lacks the lymphatic vessels that remove waste elsewhere, which long puzzled scientists. In 2012, a team at the University of Rochester proposed that in mice, cerebrospinal fluid travels along spaces surrounding blood vessels and flushes waste out of brain tissue, a mechanism they named the glymphatic system. A 2013 study by the same group reported that during sleep, the space between brain cells expanded by about 60 percent, accelerating the removal of substances such as beta-amyloid, which is linked to Alzheimer's disease.

The claim that sleep cleans the brain quickly became popular. In 2024, however, researchers at Imperial College London, using a different measurement approach, found that clearance was actually reduced during sleep and anesthesia. The original group has since reported a new mechanism in which slow, rhythmic changes in blood vessels drive fluid flow during deep sleep. Most of this evidence, though, comes from mice. In humans, imaging has mostly shown that cerebrospinal fluid moves differently during sleep, not how much waste is removed, so dramatic claims about dementia are premature.