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By James Gallagher Health and science reporter, BBC News
The brain uses sleep to wash away the waste toxins built up during a hard day's
thinking, researchers have shown.
The US team believe the "waste removal system" is one of the fundamental
reasons for sleep.
Their study, in the journal Science, showed brain cells shrink during sleep to
open up the gaps between neurons and allow fluid to wash the brain clean.
They also suggest that failing to clear away some toxic proteins may play a
role in brain disorders.
One big question for sleep researchers is why do animals sleep at all when it
leaves them vulnerable to predators?
It has been shown to have a big role in the fixing of memories in the brain and
learning, but a team at the University of Rochester Medical Centre believe that
"housework" may be one of the primary reasons for sleep.
"The brain only has limited energy at its disposal and it appears that it must
choose between two different functional states - awake and aware or asleep and
cleaning up," said researcher Dr Maiken Nedergaard.
"You can think of it like having a house party. You can either entertain the
guests or clean up the house, but you can't really do both at the same time."
Plumbing
Their findings build on last year's discovery of the brain's own network of
plumbing pipes - known as the glymphatic system - which carry waste material
out of the brain.
Scientists, who imaged the brains of mice, showed that the glymphatic system
became 10-times more active when the mice were asleep.
Cells in the brain, probably the glial cells which keep nerve cells alive,
shrink during sleep. This increases the size of the interstitial space, the
gaps between brain tissue, allowing more fluid to be pumped in and wash the
toxins away.
Dr Nedergaard said this was a "vital" function for staying alive, but did not
appear to be possible while the mind was awake.
She told the BBC: "This is purely speculation, but it looks like the brain is
losing a lot of energy when pumping water across the brain and that is probably
incompatible with processing information."
She added that the true significance of the findings would be known only after
human studies, but doing similar experiments in an MRI machine would be
relatively easy.
Brain
Commenting on the research Dr Neil Stanley, an independent sleep expert, said:
"This is a very interesting study that shows sleep is essential downtime to do
some housekeeping to flush out neurotoxins.
"There is good data on memory and learning, the psychological reason for sleep.
But this is the actual physical and chemical reason for sleep, something is
happening which is important."
Dr Raphaelle Winsky-Sommerer, a lecturer in sleep at Surrey University, said:
"It's not surprising, our whole physiology is changing during sleep.
"The novelty is the role of the interstitial space, but I think it's an added
piece of the puzzle not the whole mechanism.
"The significance is that, yet again, it shows sleep may contribute to the
restoration of brain cell function and may have protective effects."
Many conditions which lead to the loss of brain cells such as Alzheimer's or
Parkinson's disease are characterised by the build-up of damaged proteins in
the brain.
The researchers suggest that problems with the brain's cleaning mechanism may
contribute to such diseases, but caution more research is needed.
The charity Alzheimer's Research UK said more research would be needed to see
whether damage to the brain's waste clearance system could lead to diseases
like dementia, but the findings offered a "potential new avenue for
investigation".