One of the probing questions long asked by sleep researchers—why do we need so much sleep?—might finally have a more complete answer. In addition to boosting learning and memory retention and helping us feel more rested and alert, sleep gives the brain a chance to sweep up and take out the trash, a study published this week in the journal Science suggests.

To be clear, the University of Rochester Medical Center researchers performed the brain imaging study in mice, not humans, but they were able to determine that the rodents’ brains flushed out waste products much more efficiently during sleep than during periods of wakefulness. Some of the brain trash included beta-amyloid, a protein thought to be involved in Alzheimer’s disease.

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The researchers had another startling find about this waste (which is produced from the brain’s rapid conversion of glucose into energy). The reason it gets flushed so easily during sleep is that certain brain cells shrink in size, creating 50 percent more space between them for the liquidy waste to move through.

“During the daytime, only 5 percent of this fluid can move out of there; the whole system is clogged, like a Manhattan traffic jam,” said Dr. Charles Czeisler, chief of the division of sleep medicine at Brigham and Women’s Hospital, who was not involved in the study. The brain acts like a smart sanitation engineer: it’s easier to move the trash at night when the streets are clear.

Twenty years ago, another sleep researcher discovered that one of the brain’s waste products induced drowsiness and drove our biological need for sleep. The new finding adds to this previous work and might explain why larger mammals such as giraffes or elephants need only three or four hours of sleep a night, while smaller ones such as bats or mice need 14 to 20 hours.

“Larger brains should have a relatively larger volume of interstitial space to ‘buffer’ the accumulation of sleep-driving molecules, and thus might be able to withstand much longer periods of waking before the inevitable switch to the waste-clearing state of sleep occurs,” wrote Brazilian sleep researcher Suzana Herculano-Houzel in an editorial that accompanied the study.

Czeisler said he would like to see the results replicated in other mammals such as cats or dogs; it likely won’t be done in humans since the imaging test uses radioactive isotopes that doctors don’t like to administer for basic research purposes. If the results do hold up, the finding will likely prove to be a landmark one, he added, that highlights a core function of sleep and sheds light on many diseases linked to sleep disruptions.

“We’ve known that sleep can be disturbed in Alzheimer’s disease,” Czeisler said, “but this is the first suggestion that these things may be causally linked.”

He also emphasized, however, that clearing waste was likely just one of the important benefits brought on by adequate sleep. “It has nothing to do with sleep’s ability to facilitate learning or memory retention,” he added. That likely comes from associative thinking that comes from dreaming or REM sleep—towards the end of the night. The deep dreamless sleep at the beginning of the night may be the time when our brain is in full cleaning mode.