Sleep showers away cellular grime that builds up while the brain is awake — just the sort of process that could have made sleep a biological imperative, scientists reported in October (SN: 11/16/13, p. 7).
People have long puzzled over the evolutionary pressures that led animals to need sleep even though it leaves them vulnerable to predators and other dangers. Rinsing off the brain and disposing of waste proteins and other gunk might help explain why sleep evolved.
Many other things that sleep does, such as strengthening memories, are important. But they are probably bonuses to the real reason that slumber is necessary, says Suzana Herculano-Houzel of the Federal University of Rio de Janeiro.
Researchers led by Maiken Nedergaard of the University of Rochester Medical Center in New York stumbled upon sleep’s cleansing function while studying how the brain disposes of waste products.
The brain pushes fluid in between its cells to flush out buildup products, such as protein pieces that form plaques in people with Alzheimer’s disease, the team had found. After training mice to sit quietly on a microscope stage, the researchers could measure the fluid flow while the rodents were awake and asleep. Space between cells increased by at least 60 percent when the animals fell asleep, allowing cerebrospinal fluid to gush in and hose away buildup. When the animals woke up, some brain cells — probably ones called astrocytes — swelled up, narrowing the crevices separating the cells.
With the drainage system clogged, waste from hardworking nerve cells begins to pile up. Sleep deprivation or damage to the irrigation system may make it impossible for sleep to fully wash away the by-products, eventually contributing to neurodegenerative dis-orders such as Alzheimer’s and Parkinson’s diseases, the researchers speculate.