The fluid flushed out detritus twice as fast when we are sleeping as when awake, the researchers discovered.
They 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 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 understand what the function of sleep was.
"There's lots of functions that go on in a brain during sleep that can't happen [when] awake and sleep is an opportunity for them to do that," she said.
"There are certain times during sleep, certain stages of sleep, where our brain is almost as, or more, active than it is when we're awake."
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 number 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."
