Nature sub-journal: Subverting cognition! Do we really need to be "brainwashed"?

Produced by: Science Popularization China

Author: Wang Fei (PhD in Neuroscience)

Producer: China Science Expo

Editor's note: In order to decode the latest mysteries of life science, the China Science Popularization Frontier Science Project has launched a series of articles called "New Knowledge of Life", which selects cutting-edge achievements in the field of life science and interprets them in plain language as soon as possible. Let us delve into the world of life and explore infinite possibilities.

In recent years, a hypothesis of lymphatic system cleansing in the field of sleep research has attracted much attention. This hypothesis states that after falling asleep, lymphatic fluid will flow into the brain and take out the toxic substances accumulated during the day, just like the brain has completed the cleansing work.

In order to further confirm the relationship between sleep and the cleansing of toxic substances, in a recent paper published in Nature Neuroscience, scientists conducted experimental analysis and believed that the previous inference that sleep "detoxifies" was biased. So what is the relationship between sleep and the brain? How do humans "brainwash"? Let's take a look.

Is there water in our brains?

People often say "Your brain is full of water" to describe someone who is stupid or has a wrong reaction. But in reality, our brain works well because it is full of "water". Some 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. So 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. If you have ever seen a car accident, you will know that the brain splashing is not red blood flying, but more like a pool of poured tofu brain.

The reason why blood cannot directly contact the brain is because there is a blood-brain barrier between the blood vessels and the brain. This barrier ensures that the human brain, the command center of the human body, is isolated from the outside world, preventing the brain from being easily invaded by external toxic substances, and also limiting the effects of some external hormones on the brain.

Because the blood cannot provide nutrients and oxygen to the brain, the blood forms cerebrospinal fluid at the blood-brain barrier. Only cerebrospinal fluid can flow in the human brain to keep the brain working. The cerebrospinal fluid in the skull is not a stagnant pool of water, it also needs to flow continuously, flowing from the ventricles to the midbrain aqueduct.

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

The blood-brain barrier, ventricles and cerebrospinal fluid flow in the brain.

(Image source: Reference 1)

Why do we need to “brainwash”?

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 within 24 hours. So why do we need to change the water all the time? Are there fish in the water of the brain?

Yes, the fish in the brain are our brain cells. Although they cannot swim around like fish, they also excrete waste. Soon the cerebrospinal fluid is filled with toxic substances and waste produced by their metabolism.

Therefore, the brain has evolved into a type of lymphatic system, which is mainly managed by glial cells in the brain and is called the glymphatic system. This system is like installing a filter on the big fish tank of the human brain, constantly changing the water in the brain. The replacement and cleaning of cerebrospinal fluid in the brain by this glymphatic system is a kind of "brainwashing", and the human brain can stay awake only through this "brainwashing".

Although the brain only accounts for 2% of the body's weight, which is about three pounds, less than the weight of a watermelon, it consumes 20% of the body's energy, making it a large energy consumer. In the midst of so much energy consumption, the brain also produces a lot of toxic substances, such as reactive oxygen, immunoglobulins, protein fragments, etc.

Even more frightening is the deposition of beta-amyloid, a protein associated with Alzheimer's disease. The deposition of these proteins has an unclear relationship with the occurrence of Alzheimer's disease.

Although young people often stay up late, their brains don’t seem to be damaged, right? However, studies have found that although young people’s brains can recover quickly from short-term sleep deprivation, chronic and long-term insomnia and sleep disorders can cause strong stress on the brain’s metabolism . As a result, key neurons related to alertness and normal cortical function degenerate, and proteins related to aging and neurodegeneration continue to accumulate, which may make people who stay up late more susceptible to Alzheimer’s disease.

There are also studies that prove that the elderly do not often "brainwash". Their cerebrospinal fluid replacement rate is only 10% of that of young people. Young people who often stay up late are equivalent to having an old man's brain, which is full of waste that cannot be washed clean. After hearing this, will young people continue to stay up late?

Using advanced imaging techniques, researchers have discovered that the flow rate of cerebrospinal fluid increases during sleep.

(Image source: Reference 2)

When does brainwashing occur?

Brainwashing often happens at night, when we sleep. This is just like the road sweepers and garbage trucks that usually work in the early morning, evening, or even at night so as not to affect their normal work during the day.

Our brains are not stupid, of course. This cleaning system turns on at night and off during the day. In this way, brainwashing becomes a special function during sleep.

There is no consensus in the scientific community on why brainwashing often occurs at night. Recently, they have started arguing again. A previous study said that they anesthetized mice and injected tiny fluorescent tracers into their cerebrospinal fluid and found that these tracers quickly entered the brain and were discharged from the brain along specific and predictable paths.

When the mice fell asleep, cerebrospinal fluid could flow more freely, even reaching deeper parts of the brain. In the awake brain, cerebrospinal fluid can only flow along the surface of the brain, and the cerebrospinal fluid flow during wakefulness is only 5% of that during sleep. The brain clears waste products during sleep at a rate up to twice that of wakefulness.

After a series of studies on mice, the team found that when the mouse brain was in a state of sleep or anesthesia, it was busy clearing out the garbage accumulated during wakefulness. This result shocked the audience as soon as it came out, seemingly proving the hypothesis that sleep "brainwashes" for the first time.

However, "face-slapping" results also followed. An article published on May 13, 2024, by directly measuring the clearance rate of metabolites in the brains of mice in the awake, sleeping, and anesthetized states, found that the brain's clearance rate of substances would be significantly reduced in the sleep and anesthesia states.

This result is in stark contrast to previous results and suggests that excessive sleep can cause the brain to function abnormally, challenging the traditional view that the core function of sleep is to remove toxins from the brain.

Of course, scientific debates are common, and we will have to wait and see whether the rate of brainwashing during sleep is high or low. But whether the brain is "brainwashing" at night or during the day, the glymphatic system's work in cleaning brain impurities cannot be ignored.

The brain parameters are different when awake and asleep. For example, the temperature of the brain drops by two degrees during sleep and brain waves become smoother.

(Image source: Reference 3)

Sleeping well is more important

Regardless of whether we need to "brainwash" at night, the importance of sleep is self-evident. For example, dolphins and whales let the two hemispheres of the brain sleep alternately, which ensures that they can both sleep well and swim while sleeping.

If sleep is really dispensable, then why do we spend at least a third of our lives sleeping? What are the functions of sleep? In addition to helping the brain clean up metabolic waste, it can also strengthen immune function, process and store memory, etc.

But there are still many people who are sleep deprived or have sleep disorders. Being able to sleep well is a luxury for them. Studies have shown that long-term lack of sleep can cause physical discomfort, and may also induce mental problems such as trance and hallucinations, and even cause death.

It is most beneficial to master the strategy of improving your sleep. Generally speaking, sleep can be improved through proper exercise and regular diet. Many people have trouble sleeping because of insufficient exercise and insufficient gastric motility, which causes nausea and heartburn at night, affecting their sleep.

Another thing is to sleep regularly and develop good habits. Sleep is also part of the biological rhythm. The sun rises in the east and sets in the west, and the seasons change from spring to summer and from autumn to winter. These regular changes affect our organism. Sleeping at night and being active during the day are also the requirements of the biological rhythm. Regular sleep and maintaining a good biological rhythm are beneficial to our body.

I suggest that everyone let go of the stress, nothing is more important than a good night's sleep.

References:

1.Neurochem Res. 2015 December; 40(12): 2583–2599.

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