The latest paper in Nature: Is it important to have water in your head?

In recent years, there is a "glymphatic cleansing hypothesis" in the field of sleep research, which has attracted much attention in the industry. It means that after the human body enters a state of sleep, lymph fluid will flow into the brain and take out the "toxic substances" accumulated during the day, which is equivalent to a "cleansing" of the brain. In order to further confirm the relationship between sleep and the glymphatic system's cleansing of toxic substances, in a recent paper published in Nature Neuroscience, scientists conducted experimental analysis and believed that our past inferences about the "detoxification" of sleep were biased.

So, what is the effect of sleep on the brain? How do humans "clean" the brain's "cellular waste"? Let's take a look.

Is there water in our brains?

People often use the phrase "Your brain is full of water!" to describe someone who is stupid or has a wrong reaction. However, in reality, our brains can function well because they are full of "water". Studies have shown that there is 150ml of "water" in the human brain, which is equivalent to the volume of a small glass of beer. Why is there "water" in the human brain? Shouldn't it be blood? In fact, the "water" in the human brain is called "cerebrospinal fluid", a colorless and transparent liquid, and the human brain is suspended and immersed in the cerebrospinal fluid.

The reason why blood cannot come into direct contact with the brain is that blood cannot provide nutrients and oxygen to the brain. There is a blood-brain barrier between the blood vessels and the brain, which ensures that the human brain, the headquarters of the human body, is isolated from the outside world. Only cerebrospinal fluid can flow in the human brain. It flows from the ventricles to the cerebral aqueduct for normal brain function. It can not only ensure that the brain is not easily invaded by external toxic substances, but also limit the effects of some external hormones on the brain.

The cerebral ventricles and the cerebral aqueduct can actually be seen as "holes" and "pits" in the brain. There are four large "holes" in the brain, namely the first, second, third, and fourth ventricles, which are filled with "water". There is also a "pit" in the human brain that is similar to a sewer, which is the cerebral aqueduct. CSF flows out of the brain through this "pit".

Schematic diagram of the blood-brain barrier, ventricles and cerebrospinal fluid flow in the brain. Image source: Reference [1]

Why do we need to “change the water”?

The cerebrospinal fluid in the human brain is constantly flowing and being replaced. Some studies have shown that the human brain needs to replace cerebrospinal fluid four times a day in 24 hours. So why does the brain need to "change water"? Is there fish in the "water" of the brain? Yes, the "fish" in the brain are our brain cells. Although they do not swim around like real fish, they excrete "wastes", and soon the cerebrospinal fluid will be filled with toxic substances produced by metabolism.

The brain has evolved a type of lymphatic system, which is mainly managed by glial cells in the brain and is called the glymphatic system. This is equivalent to installing a filter on the "big fish tank" of the human brain, constantly "changing water" for the brain. It is precisely because of the replacement and cleaning of cerebrospinal fluid by this glymphatic system that the human brain can stay awake.

Although the brain accounts for only 2% of the body weight, which is about three pounds, less than the weight of a watermelon, it consumes 20% of the body's energy, which can be described as a "big energy consumer". A large amount of energy consumption will produce a lot of toxic substances, such as reactive oxygen, immunoglobulins, protein fragments, etc. What is even more terrifying is the deposition of beta-amyloid protein, a protein associated with Alzheimer's disease. The deposition of these proteins is inextricably linked to the occurrence of Alzheimer's disease.

But young people often stay up late, but their brains are not damaged? Studies have found that although young people's brain vitality can quickly recover from short-term lack of sleep , chronic and long-term insomnia and sleep disorders will bring strong stress to the brain's metabolism, which will cause the degeneration of key neurons related to alertness and normal cortical function, and the continuous accumulation of proteins related to aging and neurodegeneration, making people who stay up late more susceptible to Alzheimer's disease.

Studies have also shown that the replacement rate of cerebrospinal fluid in the elderly is only 10% of that in young people. In other words, if young people often stay up late, their brains are equivalent to those of the elderly.

Advanced imaging technology shows that the flow rate of cerebrospinal fluid increases when the brain is asleep. Image source: Reference [2]

When should you “change the water”?

The brain's "water change" often occurs when we sleep, just like road sweepers and garbage trucks, which usually work in the early morning, evening, or even early morning. Similarly, the brain's cleaning system is turned on at night and turned off during the day, and "water change" has become a special function when we sleep.

However, the scientific community has not yet reached a conclusion on the theoretical research that the brain often "changes water" at night. Recently, they have been "arguing" again. In a previous study, researchers anesthetized mice and injected tiny fluorescent tracers into the mice's cerebrospinal fluid. The tracers quickly entered the brain and were discharged from the brain along a specific, predictable path. When the mice fell asleep, the cerebrospinal fluid flowed more freely, even to deeper parts of the brain, while when they were awake, the cerebrospinal fluid could only flow along the surface of the brain, and the flow rate was only 5% of that in the sleeping state.

After a series of studies on mice, the team found that when the mouse brain is in a state of sleep or anesthesia, it is busy clearing the metabolic "garbage" accumulated when awake. The study showed that the speed of clearing metabolic "garbage" in the sleeping mouse brain can be up to twice that of the awake state. This result seems to be the first proof of the hypothesis that the brain "changes water" when sleeping.

However, "face-slapping" results also followed. A brief newsletter published in Nature Neuroscience on May 13, 2024, by directly measuring the clearance rate of metabolites in the mouse brain when awake, asleep, and anesthetized, found that the brain's clearance rate of substances was significantly reduced when asleep and anesthetized. This result is completely opposite to previous research results, and also suggests that excessive sleep can cause brain dysfunction. This challenges the traditional concept that "the core function of sleep is to clear toxins from the brain."

Of course, scientific debates are common, and the question of the rate of "water replacement" during sleep remains to be determined. But whether it is "water replacement" at night or during the day, the glymphatic system's work in cleaning brain impurities cannot be ignored.

The differences in brain parameters between the awake and asleep states, such as the brain temperature dropping by two degrees during sleep and the brain waves becoming flatter. Image source: Reference [3]

It is important to get a good night's sleep

In short, whether the brain "changes water" at night or not, the importance of sleep is self-evident. For example, dolphins and whales let the two hemispheres of the brain sleep alternately to ensure that they can both sleep well and swim while sleeping.

We spend at least one-third of our lives sleeping, so you can imagine how important sleep is. So, what are the functions of sleep? In addition to the above, sleep can help the brain clean up cell metabolites, strengthen the body's immune function, and help us process and store memory. However, many people in modern society have "sleep disorders".

Studies have shown that long-term lack of sleep can lead to physical discomfort, and may also induce mental confusion and even hallucinations, and in severe cases there is a risk of death.

Since sleep is so important, how can we improve it? We can take appropriate exercise and regular diet to ensure sleep. Many people have poor sleep because of insufficient gastric motility caused by lack of exercise. They are prone to nausea and heartburn at night, which affects the quality of sleep. The sun rises in the east and sets in the west, and the four seasons change throughout the year. Sleep is also part of the biological rhythm. These regular changes affect the organism. Following the biological rhythm and developing good sleeping habits are beneficial to our body.

A good sleep can keep your brain healthier. I hope each of us has a healthy body.

References

[1]Neurochem Res. 2015 December; 40(12): 2583–2599. doi:10.1007/s11064-015-1581-6.

[2]Science. 2013 Oct 18;342(6156):373-7. doi: 10.1126/science.1241224. PMID: 24136970; PMCID: PMC3880190.

[3]Science, 374,556-559(2021).DOI:10.1126/science.abi8372

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Produced by Science Popularization China

Author: Wang Fei, PhD in Neuroscience

Producer丨China Science Expo

Editor: Dong Nana

Proofread by Xu Lailinlin