A good night's sleep may be the key to preventing brain diseases such as Alzheimer's, a new study has found.
The paper, published in the journal Science, found the brain's cleaning activities increased 10-fold during sleep, helping to remove the day's toxic clutter.
Scientists had long speculated that one of the functions of sleep was to restore and repair the brain, but whether that was the core purpose of sleep remained controversial.
The researchers, led by Maiken Nedergaard of the University of Rochester Medical Centre in New York, found direct experimental evidence that brains cleaned themselves during sleep by expanding channels between neurons that allow an influx of cerebrospinal fluid.
The fluid flushed out detritus twice as fast when are sleeping as when awake, the researchers found.
The researchers found the clean-up process was so energy-intensive it would hinder our thinking if done when we were awake, the Daily Mail reported.
They concluded that unlike the rest of the body - which depended on the lymphatic system to drain toxins - the brain had its own separate method of rubbish removal.
"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," Dr Nedergaard said.
"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."
Dr Karyn O'Keeffe, research fellow at the Massey University Sleep/Wake research centre in Wellington, said the research was one more piece in the puzzle to understanding what the function of sleep was.
"There's lots of functions that go on in a brain during sleep that can't happen during wake and sleep is an opportunity for them to do that," she said.
"There are certain times during sleep, certain stages of sleep, where are brain is almost, or more active than it is when we're awake."
Sleep was important to restoring function to both the brain and the body for rest and repair, Dr O'Keeffe said.
University of Auckland senior anesthesiology lecturer Dr Guy Warman said the research was very exciting as there had been theories about why we sleep for a long time,
It was unlikely the process would affect dreams, which had more to do with sleep patterns, Dr Warman said.
"There's REM sleep and slow-wave sleep, so there's dreaming sleep and non-dreaming sleep."
An important implication of the cleaning process was if someone did not get enough sleep, the toxic build-up could mount and have health consequences, he said.
The amount of hours a person needed to sleep to avoid a chemical build-up depended on the individual, however as a general rule, eight hours of sleep each night was needed for a good, restorative sleep, Dr Warman said.
"Something so basic is really quite elegant."
Dream analyst Margaret Bowater said it was unlikely the process would affect dreams.
"We spend a quarter of our sleep's time dreaming, whatever else is going on, it doesn't stop us from dreaming."
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