If you have a pet, you'll notice how much they sleep throughout the day, especially cats. It's a mystery to us, but new research suggests the primary reason might be to allow cerebrospinal fluid to wash all the gunk from between your braincells.

Apparently, sleep strengthens links between the cells, helping turn short-term memories into long-term ones.

Researchers at the University of Rochester Medical Center weren't trying to explain sleep when they looked into the brains of mice earlier this year. Their study involved the glymphatic system, which suffuses the brain with cerebrospinal fluid, and their semi-accidental discovery only happened because they trained their test subjects very well.

The mice were trained to hang out under a microscope while the researchers probed their brains. Some of the mice were so chill about it that they actually fell asleep. That's when the team noticed a big difference between the brain of a mouse that was awake and that of a sleeping mouse.

The awake mouse had a dry brain while the sleeping mouse's brain "was almost like you opened a faucet," lead researcher Maiken Nedergaard told Science News.

This is because "junk" (fragments of proteins and other cellular detritus) accumulates between brain cells during an animal's waking hours, and less junk is around after a period of sleep. The exact mechanism hasn't been well-understood, but Nedergaard's team may have found it.

They noticed that glial cells in the brain shrink when a mouse is asleep, increasing the interstitial space between brain cells by as much as 60% allowing cerebrospinal fluid to rush in and wash away all the garbage molecules. Some of those stray protein chunks contribute to neurodegenerative disease, so the finding might lead to new research in search of cures. It also might be a more compelling evolutionary reason for sleep to exist at all – memory reinforcement might just be a beneficial side effect.

Just imagine a drug that triggers a quick glial shrinkage/cerebrospinal flush response: Instant sleep while being awake! You might not even need to sleep regularly; like new parents who struggle to get sleep.