New research suggests that sleep is the brain’s cleaning system

We spend roughly a third of our life sleeping, but even with all that 'practice', we apparently still don't know enough about it to know why we sleep. New research is giving some clues, though, and suggests that sleep may be the 'housekeeper' of the brain, cleansing it of toxins throughout the night, so that it's fresh and ready for use when we wake up.

Maiken Nedergaard, the lead researcher in the study, and Lulu Xie, who was on the research team, explain in this video:

As stated in the video, the fluid flow during sleep was much faster than while the mice were awake, which is likely due to it taking a lot more energy to pump the cerebrospinal fluid throughout the brain.

"The brain only has limited energy at its disposal and it appears that it must choice 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."

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As for what causes us to fall asleep in the first place, the research team identified a specific hormone called noradrenaline, that's more active when we're awake and less active when we're asleep. Speculation is that this hormone may be what causes the brain cells to expand while we're awake and contract when we're asleep, and the researchers believe that this discovery may someday help with the treatment of diseases like Alzheimer's.

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