Sleep Promotes Brain Health. Now We Know Why

(Posted on Wednesday, March 13, 2024)

This article is part of an ongoing series on aging and longevity. Here, we take a peek at how sleep impacts brain health and helps minimize cognitive decline. Other articles have touched on cell senescence, bone and muscle maintenance, heart health, DNA damage, and a number of other topics. 

We all know the feeling: eyelids that refuse to stay open, a yawning mouth that refuses to stay closed, and a scattered mind that refuses to stay focused. Lack of sleep is not fun. But a day lost to exhaustion may not even be the worst of it. A new study by researchers at Washington University School of Medicine suggests that sleep is crucial to “cleansing” the brain — get too little, and the brain starts to build up gunk. Published in Nature, their work highlights the nifty strategy our brains have developed to flush out debris and waste, keeping cognitive function high. Understanding this mechanism may allow for the development of drugs that mimic the benefits of a good night’s sleep.  

The Body’s Waste Disposal System(s)

Food gives us energy; that’s why we eat. The minerals and nutrients extracted during digestion are delivered to our cells, where they act as fuel. But fuel always creates a certain amount of waste: byproducts of chemical reactions that serve no particular purpose. This is no different at the cellular level. And along with metabolic waste, the body also has to contend with “debris” from cellular damage caused by aging, infection, or injury. 

Whatever the source, the obstructions need to be cleared for cells to be able to function smoothly. If left to accumulate, all kinds of important pathways and processes may become compromised. Alzheimer’s and other neurodegenerative diseases, for example, are closely associated with a build-up of cellular trash in the brain, particularly misfolded proteins. 

So how is all this waste and debris eliminated? 

In short, a specialized “sewerage” infrastructure called the lymphatic system. This is made up of various vessels and channels that, like our circulatory system, spread across the entire body. These vessels are filled with a fluid called lymph, which flows through tissues and organs, washing up and collecting debris as it goes. Dirty lymph is eventually drained into the blood vessels, where it is carried to the kidneys and liver for final filtration and removal.

The brain, being an especially active and fuel-hungry organ, creates a great deal of waste — despite accounting for only 2% of body mass, it eats up 20% of all energy. You’d expect it to have an extensive lymphatic infrastructure to match. Not so. 

For the longest time, researchers were unsure exactly how the brain gets rid of all the gunk it produces. Finally, a little more than a decade ago, a group of Danish scientists discovered the brain’s own version of the lymphatic system, which they dubbed the “glymphatic” system (the brain is filled with glial cells, which stabilize and deliver nutrients to the neurons). The researchers noticed that cerebrospinal fluid, which surrounds and cushions the brain, is flushed through the “meaty” tissue of the brain through a bespoke set of channels that run along the blood vessels. As it moves through the brain tissue, it mops up debris and waste. In a sense, it is to the brain what lymph fluid is to the rest of the body. 

Sleep Kickstarts The Glymphatic System

The central nervous system is like an electrical circuit. Each neuron, a node. This is how it sends messages across the body so quickly. Think of the last time you touched a piping-hot surface; your hand was likely moving away before you even felt the pain. That’s how quickly electrical impulses travel through the nervous system. 

The same thing happens in the brain, where one neuron will set of a chain reaction that ends in a whole network of neurons firing. These bursts of synchronized electrical impulses are called “brain waves.” Fast, high-frequency brain waves are associated with states of concentration and problem-solving. Slow, low-frequency brain waves, on the other hand, are associated with deep sleep. 

By studying sleeping mice, the team of researchers at the Washington University School of Medicine revealed that the brain waves that accompany sleep help propel cerebrospinal fluid through the deep tissues of the brain, letting it clear up any debris and waste products. When the researchers artificially silenced certain brain regions —meaning the neurons in those regions no longer fired, blocking the production of brain waves— the cerebrospinal fluid could not access the regions, and the routine cleaning was interrupted. Cellular waste quickly began to pile up. In contrast, artificially stimulating brain waves through a technique called optogenetics sets the cleansing process to hyperdrive. 

Taller brain waves, which is to say those with a larger amplitude, seem to be especially effective at flushing cerebrospinal fluid into difficult-to-reach passageways of the brain. One likely explanation is that the larger amplitude allows the waves to carry the fluid with more force. 

The scientists also noticed that the size and speed of brain waves fluctuate during sleep. Lead author of the study, Jiang-Xie, mentions that these changes may work like different settings on a dishwasher: “You start, for example, with a large, slow, rhythmic wiping motion to clean soluble wastes splattered across the plate. Then you decrease the range of the motion and increase the speed of these movements to remove particularly sticky food waste on the plate. Despite the varying amplitude and rhythm of your hand movements, the overarching objective remains consistent: to remove different types of waste from dishes.”

Implications And Takeaways

America is a chronically sleep-deprived nation. This new study is a sobering reminder of why that’s bad news: sleep helps our brains undergo routine cleaning. As we sleep, rhythmic waves flush cerebrospinal fluid deep into the brain. Once there, the fluid mops up debris and waste before being flushed out again, taking all of the gunk with it. Interfering with sleep, or not getting enough of it, risks throwing a monkey wrench into this vital sewage-disposal process. 

The findings also open the possibility of specialized treatments that mimic the effect of sleep and, in doing so, speed up the removal of waste before it has a chance to accumulate. Eventually, we may even have compounds that allow us to enjoy the benefits of a good night’s sleep without actually having to go to bed. But until then, make sure you get your sleep.