There Is a Waterway That Washes the Brain While You Sleep

Ask why we sleep, and most people answer "to rest" or "because I'm tired." That is not wrong. But sleep has one more job besides resting. While you sleep, the brain washes itself.

For people who say the morning is never refreshing no matter how much they sleep, I look at this "washing" together with them.

There Is Cleaning That Cannot Be Done by Day and Is Done at Night

The brain, too, accumulates waste when it works all day. But the brain has almost no thick vessels (lymph vessels) to carry waste away, as the rest of the body does. So how does it clear it?

At night, once you fall asleep, the brain cells shrink slightly and the gaps between cells widen. Through those widened gaps, clear fluid — cerebrospinal fluid — flows in and washes the waste away. While you are awake by day, these gaps are narrow and the fluid cannot flow well; only once you are asleep does the path open. This waterway that washes the brain is called the glymphatic system.

One thing I will say honestly here. This waterway has been observed fairly clearly in animals, but there is as yet no ready way to measure it directly in people. So what follows is less an established fact than the explanation the evidence so far allows.

So sleep is not simply a time to stop and rest, but a time to do the cleaning that could not be done by day. The deeper the sleep, the better this cleaning is done.

When the Cleaning Falls Behind, the Morning Feels Heavy

Seen this way, waking unrefreshed reads differently.

When sleep is light or you wake often, the gaps between brain cells do not open enough and the cleaning is taken to fall behind. The hours of sleep were filled, but the washing was left half-done. Then the next day the head is heavy, unclear, not refreshed. This is where the question comes from: "I slept seven hours, so why do I feel so groggy?" Because it is not only the length of sleep but how much cleaning was done within it that sets the freshness of the morning.

So for such patients I ask, before "how many hours did you sleep," "how did you sleep?" Do you wake often, do you fail to go deep, is your breathing even while you sleep? I look at whether it is sleep in which the cleaning gets done.

Hours Slept Do Not Tell How Well It Was Washed

Tests that measure sleep generally look at the hours slept and the stages of sleep. But "how well the brain was washed during that sleep" is not captured directly in those numbers. That is why "the tests say the sleep is fine, but the morning is always heavy" happens. The time of sleep and the cleaning of sleep overlap, but they are not the same thing.

But if you are so sleepy by day that you can hardly bear it, if you are told your breathing often stops while you sleep, or if you snore heavily — that may be a signal beyond this story. A test to check sleep and breathing comes first.

We tend to think of sleep as laziness or waste. But in that time the brain is quietly washing away what piled up during the day.

A morning that is not refreshing even after sleep is not from having slept lazily, but may be the body's report that the night's cleaning fell behind.

References

• While you sleep, the gaps between brain cells widen and cerebrospinal fluid washes away waste — a review of sleep and the glymphatic system — Sleep, CSF, and the Glymphatic System (systematic review), 2022
• The brain's cleaning flow changes with the sleep–wake cycle — Nature Communications, 2020

Written by Dr. Heo Ji-young (PhD in Korean Medicine Pathology, Kyung Hee University · former Research Professor of Herbology, Kyung Hee University)