The brain uses sleep to wash away the wastetoxins built up during a hard day's thinking, researchers have shown.
The US team believe the "waste removalsystem" is one of the fundamental reasons for sleep.
Their study, in the journal Science, showedbrain cells shrink during sleep to open up the gaps between neurons and allowfluid to wash the brain clean.
They also suggest that failing to clearaway some toxic proteins may play a role in brain disorders.
One big question for sleep researchers iswhy do animals sleep at all when it leaves them vulnerable to predators?
It has been shown to have a big role in thefixing of memories in the brain and learning, but a team at the University ofRochester Medical Centre believe that "housework" may be one of theprimary reasons for sleep.
"The brain only has limited energy atits disposal and it appears that it must choose between two differentfunctional states - awake and aware or asleep and cleaning up," saidresearcher Dr Maiken Nedergaard.
"You can think of it like having ahouse party. You can either entertain the guests or clean up the house, but youcan't really do both at the same time."
Plumbing
Their findings build on last year'sdiscovery of the brain's own network of plumbing pipes - known as theglymphatic 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 micewere asleep.
Cells in the brain, probably the glialcells which keep nerve cells alive, shrink during sleep. This increases thesize of the interstitial space, the gaps between brain tissue, allowing morefluid 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 possiblewhile the mind was awake.
She told the BBC: "This is purelyspeculation, but it looks like the brain is losing a lot of energy when pumpingwater across the brain and that is probably incompatible with processinginformation."
She added that the true significance of thefindings would be known only after human studies, but doing similar experimentsin 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 thatshows sleep is essential downtime to do some housekeeping to flush outneurotoxins.
"There is good data on memory andlearning, the psychological reason for sleep. But this is the actual physicaland chemical reason for sleep, something is happening which is important."
Dr Raphaelle Winsky-Sommerer, a lecturer insleep at Surrey University, said: "It's not surprising, our wholephysiology is changing during sleep.
"The novelty is the role of theinterstitial space, but I think it's an added piece of the puzzle not the wholemechanism.
"The significance is that, yet again,it shows sleep may contribute to the restoration of brain cell function and mayhave protective effects."
Many conditions which lead to the loss ofbrain cells such as Alzheimer's or Parkinson's disease are characterised by thebuild-up of damaged proteins in the brain.
The researchers suggest that problems withthe brain's cleaning mechanism may contribute to such diseases, but cautionmore research is needed.
The charity Alzheimer's Research UK saidmore research would be needed to see whether damage to the brain's wasteclearance system could lead to diseases like dementia, but the findings offereda "potential new avenue for investigation".
