It's not your fault that you're distracted after staying up late. It's actually your self - rescue mechanism "brainwashing" you.

There is further scientific evidence that staying up late can make you stupid!

The physiological mechanism for the tendency to be distracted during the day after lack of rest has been found: the brain is being "washed".

A new study from the Massachusetts Institute of Technology (MIT) has revealed the internal changes in the brain at this time. The study found that when there is a brief loss of attention, a wave of cerebrospinal fluid (CSF, which can be understood as the cleaning fluid in the brain) flows out of the brain.

This "brain - washing" process usually occurs during sleep to clean up the waste accumulated during the day. In people with sleep deprivation, this activity, which is normally restricted to nighttime sleep, is initiated during the day when they are awake - and the cost is a serious impairment of attention.

The study has been published in a sub - journal of Nature.

This study has sparked extensive discussions among netizens. Some people mentioned that they become very energetic after a short nap during the day, perhaps because their brains have been "washed" during this time.

However, the claim that staying up late makes you stupid is not a scare. In fact, there has been scientific evidence before this.

Nighttime sleep is the best time for brain - washing

In 2019, a research team from Boston University published a breakthrough result in the Science journal. For the first time, through simultaneous monitoring of functional magnetic resonance imaging and electroencephalography, they clearly captured the entire process of the human brain being cleaned during sleep.

Large - cycle blood oxygen level - dependent signal fluctuations occur in the brain. Red blood flows out of the cerebral cortex on a large scale and periodically. At this time, the blue cerebrospinal fluid quickly rushes in to fill the space left by the blood, clearing the metabolic by - products accumulated in the brain like a cleaning agent.

Among them is the β - amyloid protein, which is closely related to Alzheimer's disease.

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After waking up, the cycle disappears, and the cerebrospinal fluid cannot rush into the brain in large quantities to complete an effective cleaning.

Why can this cleaning mechanism of "blood retreats, cerebrospinal fluid enters" only be initiated during sleep?

Through monitoring 13 volunteers, the research team found the answer.

During sleep, brain neurons enter synchronized rhythmic activities (manifested as slow - wave activities recorded by electroencephalography). A large number of neurons stop firing simultaneously, reducing the demand for oxygen and glucose, allowing the blood to withdraw on a large scale. In the awake state, neuronal activities are scattered and active, and the blood needs to continuously supply energy to them, leaving no gap for the cerebrospinal fluid to enter the deep brain for effective cleaning.

So, netizens said: "It's really right to go to sleep and let your brain be washed." (doge)

This progress from Boston University shows that slow - wave loss (usually reflecting that the brain is in a state of deep sleep, brain function inhibition, or a pathological state) is related to a decrease in cerebrospinal fluid flow.

More importantly, for the prevention of Alzheimer's disease, increasing the total amount of cerebrospinal fluid for "brain - washing" will be an effective solution.

Now, the latest study from MIT shows that if you don't get enough sleep at night and the cerebrospinal fluid cannot complete the cleaning, your attention will be affected during the day.

Being distracted during the day means your brain is being washed

The research team recruited 26 healthy volunteers and asked them to participate in the experiment under two conditions: sufficient sleep and sleep deprivation.

The volunteers needed to complete visual and auditory attention tests in both states. For example, in the visual task, they had to stare at a fixed cross on the screen and quickly press a button when it turned into a square; in the auditory task, they had to identify a specific sound in a mixture of sounds and respond in time.

During the experiment, the researchers simultaneously monitored the volunteers' brain electrical signals, measured the blood oxygen saturation in their brains and the flow of cerebrospinal fluid. They also recorded physiological indicators such as heart rate, respiratory rate, and pupil diameter.

The experiment found that the performance of volunteers with sleep deprivation was significantly worse, specifically manifested as an extended reaction time. In some cases, they even showed no reaction to screen changes or target sounds and just wandered off into a blank state.

The more crucial finding was the internal changes in the brain. When the sleep - deprived volunteers were distracted, the cerebrospinal fluid flowed out abnormally; when their attention was restored, the cerebrospinal fluid flowed back.

Moreover, this process is accompanied by simultaneous physiological changes. During the period of distraction, breathing and heart rate decrease, and the pupils begin to contract about 12 seconds before the cerebrospinal fluid flows out and gradually dilate after attention is restored.

Based on the experimental data, the researchers speculated that this is the brain's "violation" operation during the day.

Normally, the brain initiates a cleaning program during the deep - sleep stage, and the cerebrospinal fluid flows regularly to remove the metabolic waste accumulated in the brain during the day to maintain normal brain function.

However, when there is insufficient sleep, this cleaning process cannot be fully carried out, resulting in the accumulation of waste in the brain. At this time, the brain acts in desperation - it forcibly initiates a cerebrospinal fluid cleaning process similar to that during sleep in the awake state.

Although this operation can remove waste to a certain extent, the cost is occupying the neural resources required to maintain attention, ultimately leading to a decline in attention.

That is to say, even when a person is awake, the brain will forcefully initiate a sleep - like cleaning process, occupying the "energy" originally used to maintain attention and causing distraction.

In addition, the study also speculates that there may be a master switch in the brain that controls both our attention and physical responses such as cerebrospinal fluid flow, heartbeat, and breathing. A substance in the brain called "norepinephrine" may control this switch, but it has not been fully confirmed yet.

The first author of this study is Zinong Yang, a computational neuroscience researcher who mainly studies brain dynamics under different attention states.

She obtained a bachelor's degree in cognitive science from the University of San Diego and later a doctorate from Boston University and MIT, under the supervision of Laura D. Lewis.

Both of the above studies are from the Lewis Lab, and the corresponding author is Laura D. Lewis.

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So, situations like swiping your phone to look up information and suddenly forgetting what you were searching for may not just be due to lack of energy. Instead, it may be that your brain is "violating the rules" and initiating a brain - washing program after staying up late.

Let's try not to stay up late in the future.

Reference links:

[1]https://www.nature.com/articles/s41593-025-02098-8

[2]https://news.mit.edu/2025/your-brain-without-sleep-1029

[3]https://news.ycombinator.com/item?id=45771636

[4]https://science.sciencemag.org/content/366/6465/628

This article is from the WeChat public account "QbitAI", author: Wen Le. Republished by 36Kr with permission.