The Nuance

Why Do We Sleep? Science May Have Finally Figured It Out

Your brain is equipped with a waste management system that does most of its work while you slumber

Markham Heid
Published in
4 min readJan 19, 2022
Photo: Lux Graves/Unsplash

Two centuries ago, the Scottish physician Robert Macnish theorized that the purpose of sleep was to “renovate the mind” by offering it a period of deep repose.

The idea that sleep helps rejuvenate a weary brain had been around long before Macnish’s time. But as recently as 20 years ago, sleep scientists still readily admitted that they did not understand the fundamental purpose of sleep.

For decades, we’d recognized that some important biological processes take place during sleep, and that a lot goes wrong with us when we don’t get our ZZZs. But none of this explained just why human beings — and pretty much every other type of beast, bird, or bug on Earth—spends such a large portion of their lives in slumber.

The discovery of a hidden brain system may finally provide the answer.

The metabolic and cellular processes that keep you alive are not perfectly efficient. They produce waste.

Fortunately, your body is equipped with a kind of waste management system. It’s called the lymphatic system, and its network of fluids, nodes, vessels, and organs collects and removes all the cellular junk — as well as the bacteria, proteins, and any other unwanted detritus — that builds up inside of you.

“The universal biological need for sleep across multiple species may, at least in part, reflect the need for glymphatic clearance.”

We’ve known about the lymphatic system since the late 1700s. But until very recently, experts had failed to locate an equivalent system in the human brain.

That changed a decade ago when a group of researchers, based mainly at the University of Rochester Medical Center in Rochester, New York, discovered a previously unknown “clearing system” in the brain. They called it the glymphatic system, and they showed how it helped collect cellular waste and flush it out of the central nervous system.



Markham Heid

I’m a long-time contributor at TIME and other media orgs. I write mostly about health. I grew up in Michigan, but these days I live in southwest Germany.