Scientists in the United States believe they may have uncovered one of the fundamental reasons for sleep.
Researchers at the University of Rochester say experiments on mice have shown that the brain uses sleep to flush out waste toxins which accumulate during waking hours.
As the brain spends the day working hard and analysing the world, there is a build-up of waste material. The proteins and chemicals produced as neurons communicate can eventually become toxic, the BBC reports.
Experiments on mice show that brain cells shrink during sleep to open up tiny gaps in the organ. This allows fluid to be pumped through and wash waste material away.
The researchers said the brain was acting like the host at a house party - it could entertain the guests or clean up, but didn't have the energy to do both at the same time.
Their study has been published in the journal Science.