While you sleep, your brain rinses itself.
In 2013, researchers found that in sleeping mice the spaces between brain cells widened by about 60 per cent, letting cerebrospinal fluid flush through and wash out waste, including proteins linked to Alzheimer's. Sleep, it turns out, is partly housekeeping.
Most organs clear waste through the lymphatic system, a network of vessels that drains fluid from tissues. The brain has no conventional lymphatic vessels inside it, which left a puzzle: how does such a hard-working organ take out its rubbish?
In 2012, Maiken Nedergaard's lab described what it called the glymphatic system. Cerebrospinal fluid flows in along channels around the brain's arteries, moves through the tissue and carries waste out along the veins. Cells called astrocytes, with water channels called aquaporin-4, help drive this flow.
A year later, the same group published a striking follow-up in Science. In sleeping or anaesthetised mice, the space between brain cells expanded by about 60 per cent, and the flushing of fluid increased dramatically. Beta-amyloid, a protein that accumulates in Alzheimer's disease, was cleared about twice as fast during sleep as during waking.
The findings suggest that one reason we sleep is to clean the brain, a job that may be hard to do while it is busy thinking. Much of the evidence still comes from mice, and scientists continue to debate how the system works in people and how it varies with sleep stage. But it adds a vivid reason to protect deep sleep: part of what feels like rest is the brain doing its washing up.
Source: Xie et al. (2013), Science
