Why do we sleep? To clean our brains, say US scientists

Scientists in the US claim to have a
new explanation for why we sleep: in
the hours spent slumbering, a rubbish
disposal service swings into action
that cleans up waste in the brain.
Through a series of experiments on
mice, the researchers showed that
during sleep, cerebral spinal fluid is
pumped around the brain, and
flushes out waste products like a
biological dishwasher.
The process helps to remove the
molecular detritus that brain cells
churn out as part of their natural
activity, along with toxic proteins
that can lead to dementia when they
build up in the brain, the researchers
say.
Maiken Nedergaard, who led the
study at the University of Rochester,
said the discovery might explain why
sleep is crucial for all living
organisms. "I think we have
discovered why we sleep,"
Nedergaard said. "We sleep to clean
our brains."
Writing in the journal Science ,
Nedergaard describes how brain cells
in mice shrank when they slept,
making the space between them on
average 60% greater. This made the
cerebral spinal fluid in the animals'
brains flow ten times faster than
when the mice were awake.
The scientists then checked how well
mice cleared toxins from their brains
by injecting traces of proteins that
are implicated in Alzheimer's disease.
These amyloid beta proteins were
removed faster from the brains of
sleeping mice, they found.
Nedergaard believes the clean-up
process is more active during sleep
because it takes too much energy to
pump fluid around the brain when
awake. "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," she said in a
statement.
According to the scientist, the cerebral
spinal fluid flushes the brain's waste
products into what she calls the
"glymphatic system" which carries it
down through the body and
ultimately to the liver where it is
broken down.
Other researchers were sceptical of
the study, and said it was too early to
know if the process goes to work in
humans, and how to gauge the
importance of the mechanism. "It's
very attractive, but I don't think it's
the main function of sleep," said
Raphaelle Winsky-Sommerer , a
specialist on sleep and circadian
rhythms at Surrey University. "Sleep
is related to everything: your
metabolism, your physiology, your
digestion, everything." She said she
would like to see other experiments
that show a build up of waste in the
brains of sleep-deprived people, and
a reduction of that waste when they
catch up on sleep.
Vladyslav Vyazovskiy , another sleep
expert at Surrey University, was also
sceptical. "I'm not fully convinced.
Some of the effects are so striking
they are hard to believe. I would like
to see this work replicated
independently before it can be taken
seriously," he said.
Jim Horne, professor emeritus and
director of the sleep research centre
at Loughborough University,
cautioned that what happened in the
fairly simple mouse brain might be
very different to what happened in
the more complex human brain.
"Sleep in humans has evolved far
more sophisticated functions for our
cortex than that for the mouse, even
though the present findings may well
be true for us," he said.
But Nedergaard believes she will find
the same waste disposal system at
work in humans. The work, she
claims, could pave the way for
medicines that slow the onset of
dementias caused by the build-up of
waste in the brain, and even help
those who go without enough sleep.
"It may be that we can reduce the
need at least, because it's so annoying
to waste so much time sleeping," she
said.