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," Nedergaard said in a statement. "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."
For instance, the protein beta-amyloid, which is linked with Alzheimer's disease, flowed out of the brains of sleeping mice twice as fast as it flowed out of the brains of awake mice. Overall, the flow of waste out of the brain while awake was only 5 percent of what it was when mice slept.
Also, the researchers surprisingly found that cells in the brain shrink by 60 percent during sleep. This creates more space between the cells, helping waste to wash out the brain's plumbing more effectively.
The scientists noted that a hormone called noradrenaline is less active in sleep. This chemical is released in bursts when the brain needs to become alert, often in response to threats. The research team speculated noradrenaline might help control how brain cells expand and contract during sleeping and waking.