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.
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".
http://www.bbc.co.uk/news/health-24567412
