The brain uses sleep to wash away the waste toxins built up during a hard day's thinking, researchers have shown.
The US team believes 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."
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